What's the Greek manuscripts of John with the oldest discovery date?

John, could you please specify what "recovery date" means?

The reason I ask is that, for a number of these mss handed down to us, there are multiple recovery dates. Take Codex Alexandrinus as a case study:

Codex History Prior to 1627 Prior to 1627, the history and provenance of Alexandrinus remain somewhat obscure. The codex was certainly in the hands of Lucar in Constantinople, where he was transferred after being elected to the patriarchate in 1620. In light of the inscription on V1.F5a (discussed below), it is probable that Lucar brought Alexandrinus with him from Alexandria when he made the journey to Constantinople.20

However, early data regarding Lucar's acquisition of Alexandrinus were also provided by Johan Jacob Wettstein, a Swiss chaplain serving in the Dutch army. Wettstein had worked with Richard Bentley and had delivered his collations of various New Testament texts to Bentley in 1716, hoping to encourage the scholar to publish his own edition of the Greek New Testament (which Bentley never completed).21 Undertaking the work himself, Wettstein discussed in the prolegomena of his 1751 Greek New Testament a history of Alexandrinus that he discovered in familial correspondence. Referring to two letters from his great-uncle, J. R. Wettstein, to Martin Bogdan (dated January 14 and March 11, 1664), Wettstein revealed that his great-uncle reported the witness of a Cyprian named Matthew Muttis, a deacon of the patriarch. According to Muttis, the codex was found at Mount Athos (northern Greece), in a monastery that escaped Turkish persecution through paying tribute.22 Despite assumptions to the contrary, Muttis did not explicitly claim that Cyril Lucar found the manuscript.23 In the second letter, the elder Wettstein also related that while royal librarian and patristics scholar Patrick Young was preparing an edition of Clement of Rome from Alexandrinus, a fire at the royal museum burned the book and created scorched lacunae in the text; the manuscript was only saved after being thrown from a window during the fire.24

The plausibility of the account given by Muttis will be evaluated after additional data are examined. Since the nineteenth century, debate regarding the origins of Alexandrinus and the hands it has passed through over time has cen- tered on a few Arabic and Latin marginalia added to the text at a later date. The inscriptions/interventions that follow will be addressed in turn, noting how each has been interpreted chronologically. Inscription on V1.F4a

On a flyleaf of the first volume of the text, a Latin note appears in a neat hand reading: "donum datum cubiculo Patriarchali anno 814. Martyrum."25 Below this note and in a second hand, in pencil and in larger text-is written: '+ AD 284 = 1098." As the date of the first note is according to Anno Martyrum, it is measured according to the Coptic calendar, which begins with the reign of the Roman emperor Diocletian (under whom many, especially Egyptian, Christians were tortured and killed) in AD 284. Thus, this note asserts that the text was given to the patriarchal cell in AD 1098, as the second hand calculates. This note has been unanimously determined to be late, added after Alexandrinus passed from Cyril Lucar's hands.26 In 1826, Baber suspected that the note was written not long after the codex arrived in England.27 This inscription is estimated by Edward Maunde Thompson in 1881 to be "of the latter part of the 17th century."28 In 1926, Foakes Jackson and Lake were in agreement with this dating, noting that "the source of this information (or conjecture) is not known."29 Scot McKendrick pointed out in 2003 that this inscription was most likely "an inaccurate attempt at deciphering the Arabic note by Athanasius”30 found on V1.F5.1

Notice the chain of possession:

It is an ancient uncial copied in the 'golden age' of textual transmission after Christianity was a "religio illicita" and before the Mohammedan conquest

It was, perhaps, given to the Patriarchate cell in 1098

The earliest certain possession, was when it was given to King Charles I of England by Cyril Lucar (Patriarch of Constantinople (1621-1638))2

Alexandrinus survived not one, but two fires. And ironically, we would not know as much about its history, if it hadn't been rescued and then moved somewhere else on several occasions.

This sort of history of manuscripts isn't all that uncommon. You most likely won't find a reliable chain of possession and discovery simply because Christianity was a "religio illicita" till Constantine. And, with the Muslim conquest, Christians were persecuted and their bibles were burnt.

There is a reason why they determine the age of manuscripts not primarily due to "discovery dates". Trusting the chain of discoveries is messy. But, in most instances, it's not needed. Paratextual features and paleography determine the date of copying in a more reliable manner.

And also, as a correction, Vercelli (it^a) would not qualify, since you specify greek witnesses (Vercelli is Old Latin).

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1 A study in the gospels in codex Alexandrinus, Brill, 2014, Andrew Smith, pp. 12-14.

2 Smith, p. 8

My hunch is that you'll find older manuscripts in religious institutions rather than academic libraries. Those housed in national or academic libraries were frequently purchased by archaeologists, travelers etc. in the past few hundred years (e.g. the Oxyrhynchus Papyri or Bodmer Papyri). But you can't "discover" something if the institution never forgot they had it.

I'd wager the Codex Vercellensis is your best bet. It's been housed in the Capitulary Library of Vercelli (continuously?) since the 4th century, when it was commissioned by St. Eusebius of Vercelli.