Who First Identified that Beza's Conjecture at Revelation 16:5 was a conjecture?

 A reader recently emailed with a question, which I am paraphrasing in the following way:

Who is the first person to claim that Beza was engaging in conjectural emendation by replacing hosios with esomenos? 

This is a difficult question. 

As early as 1764, in Anthony Purver's "A New and Literal Translation of the All the Books of the Old and New Testament; with Notes Critical and Explanatory," vol. II, p. 332, we find the following:

"Ver. 5 holy] shalt be only on Beza's slender Authority, that being not in all the various Readings of Mills, Kuster, Wetsten, Etc. though Doddr. says other Copies have it so, which however he does not follow. By Wesl. the Gracious one, but wrong.

In 1861, William Webster's "The Greek Testament with Notes Grammatical and Exegetical," vol. II, p. 816 states:

A.V. has ' and shalt be ; ' reading ὁ ἐσόμενος from Beza on insufficient authority 

Saying, "Beza's slender authority" and "on insufficient authority" are not quite fully saying that the conjecture was a conjecture.  The reason for hesitancy was, of course, that Beza's own annotations most naturally indicate that Beza had amended in some way based on a manuscript.

In 1881, "The Holy Bible ... with Explanatory and Critical Commentary and a Revision of the Translation ..." (Cook, ed.) (p. 463)(similarly, in the 1890 printing, p. 463), states:

The third edition of Stephens (A.D. 1550) was the basis both of the editions of Beza (Geneva, 1559, 1565, 1582, 1589, 1598,-- see Scrivener l. c. p. 390) and of the Textus Receptus. Beza's edition of 1589² (or 1598) was taken as the basis of our Authorized Version, by the Translators of 1611:  and thus the English Version of the Apocalypse represents a Greek text which does not rest upon the same authority as that of the other Books of the New Testament³ E.g. in ch. xvi. 5, the conjectural reading of Beza's last three editions (ἐσόμενος for ὅσιος, which rests on no authority whatever) is still represented in the words of the Authorized Version "and shalt be." 

This is one of the earliest English-language references to Beza's conjecture, as such, but the study of the New Testament is not limited to English. 

In 1833, Commentationes Latinae, Tertiae Classis, Instituti Regii Belgici, Volumen Quartum, (Disputation with Valckenaerii, p. 17) says:

c. XVI. 5. Καὶ Ὁ ἐσόμενος Ende Die zijn fal. Complutenses, ERASMUS et STEPHANUS, ediderant, quod omnes agnoscunt versiones, et codices Gr. fere omnes: καὶ ὁ ὅσιος, estque ea lectio, ab ELZEVIRIIS recepta, huic loco valde apta. Recte igitur eam, in I. et II. editione, secutus fuerat BEZA: sed in III. IV. et V. eius in locum substituit lectionem a PISCATORE et BELGIS praelatam, hac addita annotatione: Legitur vulgo καὶ ὁ ὅσιος, ostendente articulo, praeter omnem loquendi morem, depravatam esse scripturam. Vulgata vero, sive articulum legit, seu non legit, nihilo rectius vertit ὅσιος, sanctus, male extrita particula καὶ, prorsus necessaria, ut δίκαιος et ὅσιος connectantur. Sed quum Johannes reliquis omnibus locis, ubi Jehovae nomen explicat, sicuti diximus supra 1. 4. addere consueverit tertium, nempe καὶ ̔Ὁ ἐρχόμενος, cur istud h. 1. praeteriisset? Itaque ambigere non possum, quin germana sit scriptura, quam ex vetusto bonae fidei manus. codice restitui, nempe Ὁ ἐσόμενος. 

Haec BEZA, quibus vero hodie nemini persuadebitur. Nec persuasum fuit ROLANDO, qui Graece scripsit: καὶ ὁ ὅσιος, Belg. en die heylige. Eamque germanam lectionem, in veteri quoque belg. versione expressam, in notis commemorant BELGAE. Intelligitur autem e BEZAE annotatione, alteram lectionem, unius codicis auctoritate, non autem, ut VALCKENAERIO visum fuit, (p. 334.) nuda coniectura assumtam esse a nostratibus. Cf. ELUIT, p. 703.

The reference to "Valckenaerio" seems to refer to Lodewijk Caspar Valckenaer, a Dutch classicist (1715-1785).  I could not immediately locate the specific printed edition (or maybe not even the correct work), but I did find the same point expressed here:

Apocal. c. XVI, 5, in istis, ὁ ὢν, καὶ ὁ ἦν, καὶ ὁ ὅσιος⋅ legendum καὶ ὁ ἐσόμενος, pro ὅσιος, suspicati sunt Piscator et Beza: vera suspicio visa fuit Belgis Interpretibus; dederunt enim: die is, ende die was, ende die zyn sal. Suam, ut puto, coniecturam expresserunt eiusdem 

Ludovici Caspari Valckenaerii Opuscula philologica, critica, oratoria nunc primum coniunctim edita ; accedunt indices · Volume 2, (p. 241)

Update: I found what I believe to be the actually cited work, which as the same statements: (p. 334)

Tiberus Hemesterhusii et L.C. Valckenari Orationes (1784)

I'm not sure who literally is the first who came out and said that he believed it to be a conjecture, in so many words. However, I don't claim to have done an exhaustive research of the topic.

*** 

Same Day Update:

The "NT Conjectures" page notes a variety of historic comments about Beza's conjecture (link to page). That list does not (at the time of this post) mention Valckenaer, but it does mention  Johann Christoph Wolf, Curae 4 (11735), 566–567, which says (in 1735):

καὶ ὁ ἐσόμενος Beza: ex vetusto, ut ait, bonae fidei ms. codice. nec tamen nihil dedisse coniecturae videtur. cur enim in nullo alio Apocalypseos loco illum codicem annotationes Bezae memorant? aut cur Rob. Stephani margo nil hoc loco variat? Bezam tamen sequitur H. Stephanus, Pareus, Piscator, Amama: ac plenius. adhibita parenthesi, (καὶ ὁ ἐσόμενος,) καὶ ὅσιος E. Schmidii textus habet. sed Bezam reprehendit Marckius ad h. l. et gravius Zeltnerus in Diss. de Evangelio Tetragrammato p. 29.

καὶ ὁ ἐσόμενος  Beza: from an ancient, as he says, manuscript of good faith. Nor, however, does he seem to have contributed nothing from conjecture. For why, in no other place of the Apocalypse, do Beza’s annotations mention that codex? Or why does Robert Stephanus’ margin vary nothing in this place? Yet H. Stephanus, Pareus, Piscator, Amama follow Beza: and more fully, with the parenthesis applied, (καὶ ὁ ἐσόμενος,) καὶ ὅσιος the text of E. Schmid has. But Marckius criticizes Beza at this place, and more seriously Zeltner in the Dissertation on the Tetragrammaton Gospel, p. 29.

So, this is actually an earlier example than what I provided in the original post. 

Also we find Adriaan Kluit, Vindiciae 1.3 (1771), 703–704 , which says (in 1771):

De Recepta Lectio heeft: δίκαιος, Κύριε, εἶ, ὁ ὤν, καὶ ὁ ἦν καὶ Ὁ ὍΣΙΟΣ—. Gij zijt rechtveerdig, heere, die is, en die was, en de heilige, dat gij dit geoordeeld hebt. Maar de Ned. Overz. — die is en die was en die zijn zal, dat gij — Kantteekening: “Andere lesen, ende de Heylige. —” Doch deze aanteekening voldoet geenzins. Want niet alleen, dat andere lezen de heilige, maar ’t is opmerkelijk: daar is noch nooit een eenig MS. vertoont, waarin dit onzer Nederd. Overz, die zijn zal, ὁ ἐσόμενος, te vinden is. En niet alleen dit, maar zelfs alle Uitgaven, van eerst tot lest hebben ὅσιος, heilige, ’t is alleen beza, die in Edit. 3–5. zich de vrijheid aangematigd heeft om hier ὁ ἐσόμενος te zetten, gelijk Wolff h. l. toont; maar de Recepta Lectio behoudt noch heden bij allen ὁ ὅσιος. De meeste MSS. leiden ons intusschen daarhenen, om hier te lezen eenvoudig: Δίκαιος εἶ ὁ ὢν καὶ ὁ ἦν ὅσιος, ὅτι ταῦτα ἔκρινας· of ὁ ἦν, ὁ ὅσιος, gelijk H. I, 8.

“The Received Reading has: δίκαιος, Κύριε, εἶ, ὁ ὤν, καὶ ὁ ἦν καὶ ὁ ὅσιος—. ‘Thou art righteous, Lord, who is, and who was, and the Holy One, because thou hast judged these things.’ But the Dutch Version: — ‘who is and who was and who shall be, because thou—’ Marginal note: ‘Others read, and the Holy One.—’ Yet this note by no means suffices. For not only that others read ‘the Holy One,’ but it is remarkable: there has never yet been produced a single manuscript in which this, of our Dutch Version, ‘who shall be,’ ὁ ἐσόμενος, is to be found. And not only this, but even all editions, from first to last, have ὅσιος, ‘holy’; it is only Beza, who in Editions 3–5 has assumed the liberty to put here ὁ ἐσόμενος, as Wolff in this place shows; but the Received Reading still retains with all, ὁ ὅσιος. The majority of manuscripts, meanwhile, lead us thither, to read here simply: Δίκαιος εἶ ὁ ὢν καὶ ὁ ἦν ὅσιος, ὅτι ταῦτα ἔκρινας· or, ὁ ἦν, ὁ ὅσιος, just as [in] I, 8.”

This comes very close to saying that it is a conjecture, and clearly endorses Wolf, who does say that.

(Thanks to Nick Sayers, who reminded me about the NT Conjectures page.)