I agree with CaptSteve. I owned a GGT with 4 holes and assumed Imeo made them. The center hole appeared machined made and seemed to be same size as the other 3 holes, although I didn't measure it.
I've come across several GGT's with 4 holes besides mine (not in person - but pics on the net). If 10 or 20 people with OCD created 4 hole GGT's by over buffing, it leaves me wondering why I've never seen a 4 hole brass bottom cap.
Also, if a person is in fact consumed with buffing bottom caps, they'd also buff the hell out of the tube and GG engravings would vanish. My GGT still had the logo and machined lathe markings, which wasn't uncommon for it back then.
Another twist is the GGT in Dave's first post and mine both had aluminum bottom caps. I know some were made, and brass later came into fashion as people liked the accent. I believe many GG Grants had aluminum bottom and top caps.
Guess it's a mystery and a great conversation piece!