I just got a Fatboy with a G3 date code and a Gold slim with a G4 date code. Both made in 1961. Am I right in assuming the Slim came out in the fourth quarter of '61?
Yeppers,
Have a look at the wiki
http://wiki.badgerandblade.com/index.php/US_Gillette_Dating_Information
Appears there was one years overlap in production 1961 ( G )
So yes
FB G3 - 1961 3rd quarter
Slim G4 - 1961 - 4th Quarter