Very little Mtarfa turns up in any reference books, far less images of labels, I'm afraid.
It is likely that there was even less information about it around when Ivo wrote the book (Glass Fact File a-z was published in 2001.)
In 2000, Lesley Jackson published 20th Century Factory Glass, and in that, there was the very first information I found published on Mdina (apart from a single black and white thumbnail image in an old Miller's Guide, prior to that) - and that was under Isle of Wight Studio Glass, not meriting a section on its own even then. There is a mention of Mtarfa there, but no dates.
Mdina was discussed in the Ronald Stennet-Willson book, in (I think) 1975, but that went out of print long before I could get hold of a copy.
Maltese glass is has only really become popular in the last 10 or so years, there is very little about it in books.