Yes, oddly enough, in the earlier Electros (pre-G through GS/GT?) the battery covers (with the spanner holes) measure 20.9mm across overall and 19.8mm across the threads. In the GSN/GTN series (without the spanner holes) the covers measure 20.5mm overall and 19.5mm across the threads.
I don't know for sure that the change in size didn't occur at some point within a model (late GS/GT or early GSN/GTN), but I do know that my pre-G and my GT use the same size, larger, cover, but both my GSNs have the smaller cover.
I wonder why Yashica felt it necessary to make that tiny change.
Patrick, my offer of a GSN cover is still good. Sorry, Tfishmrt, I don't have a spare GS/GT cover.