Many thanks indeed to Diego and Matthieu Cassin who both contributed a lot of useful information in response to my last post. Let me summarise their comments.
It seems that the K. Hadjioannos (1975) edition of Philo of Carpasia’s Letter to Eucarpios is a straight reprint of a text printed by A. Papadopoulos-Kerameus’ in Analekta Hierosolymitikes Stachyologias, vol. 1 (1891), item 16, 393-399, minus the apparatus and biblical references. His text was taken from MS St Sabbas 408 (9-10th c.), folios 34-40. Interestingly the manuscript leads off with two works by a certain John Carpathius.
The Analekta volume is a collection of texts printed from the Greek Orthodox Patriarchate in Jerusalem and its churches in the surrounding area. Papadopoulos-Kerameus in fact made a detailed catalogue of the manuscripts at the Patriarchate, which includes manuscripts from the monastery of St Sabbas (or Mar Saba as it is often known). The Mar Saba manuscript appears in this catalogue in vol. 2 (1894) p.536, entry 3 (online here), although I learn from Pinakes that more up-to-date catalogues also exist. Apparently microfilms of around half the manuscripts at the patriarchate were made in 1950 by an American expedition.
The marvellous Pinakes database also lists another manuscript of the Letter to Eucarpios here. It is in Greece, on Mount Athos, in the Iviron monastery. It is Iviron 673 (numbered 4793 in the Lambros catalogue), and dated to the 14th century. Here the Letter follows two works by Chrysostom, so clearly the Mar Saba manuscript is not just a straight copy of the Iviron manuscript.
I’ve begun to scan the modern Greek translation of the Letter to Eucarpios. I don’t know a word of modern Greek, but I thought it possible that Google Translate or ChatGPT might be able to do something with it. Google Translate certainly attempted it, but the results were not very good. ChatGPT on the other hand has given a very fluent translation of the whole thing, which I will post once I have tinkered with the output a bit. At one point in particular the thought is obvious but the raw output obscures it.
The letter is written to a monk who is just starting out on the ascetic life and is despairing, and wondering, “what’s the point?” The response is more interesting than you might imagine.