GCFCG
P.O. Box 683
Columbia, MD 21045
As I reecall, we declared ourselves the founding chapter because we were the only ones with 501(c)3 status—which we inherited from CCC3—and for a good many years the ICG piggybacked on our 501(c)3 status (while other chapters piggybacked on the ICG).
We noticed that some other chapters had nicknaes, like the Sick Pups and the Costume Mafia, but we had none. When the ICG eventually filed for its own 501(c)3, we had a vote that declared our nickname was “the founding chapter.” At the time, Star Trek DS9 featured a race called The Founders, so we also voted that henceforth, GCFCG would be pronouunced “Founders.”
So the whole Founnding Chapter/Founders thing is a classic case of what math nerds call Proof By Blatant Assertion: We're the Founders because we say so.