У Алана Келли указан матричый номер 2906a - какая то здесь путаница с номерами...

In articles by Alan Kelly and others, I have read that the series used for each record size were rationalised, I think about 1903-4.
Before this the recording experts often used their initials with or without other letters.

Later editions of records use the rationalised numbering scheme, in which Fred Gaisberg used a for 7 inch, b for 10 inch and c for 12 inch. Will Gaisberg used d e and f for the three sizes I think. I think it may have been Alan Kelly's policy to use the later numbers.

It looks like "fg" stands for Fred Gaisberg...

Yes it will be for Fred Gaisberg. Will used WG, WCG etc. nB was another suffix used by Fred. As you might guess, the whole picture is complicated, but after early 1904 a, b and c are used by Fred and his assistants, d e and f by Will*, and so on. These continued in the same series that had the old prefixes and suffixes and later pressings of earlier recordings tend to use the new suffix.

The full details are in the Record Collector, Vol XXIII, Nos 3 & 4 in an article by Perkins Kelly and Ward.

*except that some WG (Will) matrices were in the a b c series... I told you it was complicated.

Thank you, Adrian!