Thirty
Mark Liberman at Language Log meets thirty for the first time, and restricts it too narrowly to newspapers although the earliest citations don’t mention them:
1895 Funk’s Standard Dict., Thirty..among printers and telegraphers, the last sheet, word, or line of copy or of a despatch; the last; the end. 1929 Amer. Speech IV. 290 ‘30’ or ‘Thirty’ indicates the end of a shift or of the day’s work, and has come to mean, also, death.
The XXX theory of thirty seems the most likely, for its similarity to ###.