I would imagine that the Orthodox priest, his interlocutor, or both, misinterpreted what they were seeing.
As you know, the Church has a concept of "partial indulgence", historically often associated with a specific amount of time. The intention, as I understand it, is that gaining the indulgence corresponds not to that amount of time off Purgatory, but to a remission equivalent to what one would get from performing "old-school" penance for that duration. In effect, the counts in days and years are units of measure of the penance that is being done, not the results. I have also seen indulgence lists where the measure was given not in terms of time, but something like "all the remissions one would gain if one were to make a pilgrimage to the shrine of St Genericus in Exampleton".
Lists of indulgences have been published for a long time. There are books that reproduce every prayer, etc., in full detail. I have also seen leaflets and prayer cards which give very abbreviated reminders, like "Benedictus qui venit - 500 days". The big books will obviously include much more explanation of the doctrine. If summaries of this kind were printed in a Bible, or a devotional book other than a Bible, in early 1900s Ireland, I'd assume that such a book would only be produced with the nihil obstat and imprimatur of the local bishop. That approval would certainly be required from the perspective of the Church, and anyone producing a book for Catholics in Ireland would be pretty motivated to follow the rules. I don't know what process is required for more ephemeral materials, but in that case it's certainly more likely that someone would just print the thing on their own account.
Therefore, I think that any official list of this kind would not be intended to refer to "time off Purgatory". That does not stop it being read that way, especially if it's one of the highly abbreviated reminder lists. I imagine that this sort of confusion lies behind the abolition of temporal measures for partial indulgences (Paul VI, Indulgentiarum Doctrina, 1967; section 12 and norm 5).
In this case, I suspect that the "list" may not have been an actual list, but just a reproduction of the specific indulgences related to the reading of Scripture. I have found a few Catholic Bibles on Google Books which include this text; for example, a 1914 Douay-Rheims Bible from the USA. After the approbations by three American cardinals, and before the preface and table of contents, there is a single page with the following text:
An indulgence of three hundred days is granted to all the Faithful who read the Holy Gospels at least a quarter of an hour. A Plenary Indulgence under the usual conditions is granted once a month for the daily reading. 13 December, 1898. LEO XIII.
This reference must be to the edition of Enchiridion indulgentiarum, the official indulgence manual, published on that date. There is no further explanation of the doctrine of indulgences, the "usual conditions", and so forth. It is followed by a 1778 letter of Pius VI commending the translation of the Bible into the vulgar tongue, and a suggested prayer for the reading of Scripture.
It seems quite likely to me that the Bible in question was one like this, and the reference to "three hundred days" - in the absence of any text explaining the doctrine - was misinterpreted by the readers.