There are presently some Frankford Model of 1909 .45 Cartridges listed on an Auction Site who's initials would be something like 'gb'.

Search term '45 Colt Model 1909 Frankford" would likely bring it up, since that is the listing title phrase.

I know these are not the 'mystery' Cartridge in question, but, it is kind of fun to see what the '09 looked like anyway.

I have no affiliation with the seller and I am not promoting the item.