You aren't alone, this stuff is fascinating. Fullgraff must have worked for Brett to be speaking for them in contracts and using their letter head, but he also worked for whoever Old Masters is at this time. I don't have much in the wya of books or documents on Lithographers, but I will keep digging and see if I can find anything to tie these firms to American Lithographic or disprove a connection.
I found a June 1904 copy of the New York Athletic Club's Journal (
https://books.google.com/books?id=yz...20club&f=false, page 34). He was apparently into boating as well. Harry Hillman, a T218 subject, is in the issue as well.
The Larchmont Yacht Club and the Columbia Yacht Club, both of which he was a member of in 1904 according to this, are found in the 4th series of T59 flags.
It seems many of these T cards, among popular subjects and figures of the day, featured his friends and associations.
These NYAL journals are probably a good source on some of his activities. I believe the NYAL still exists and may have something in their records on Mr. Fullgraff.