Thank you, I was hoping for an answer some time before September...:wink:
It comes with experience. There are many slight nuances. And for example there was a lot of cross breeding of components between the GC-I and GC-11. GC-3 and beyond not so much. GC-I and GC-II are wery similar. Same base, legs and stretcher construction. Look at the stretcher and if it's old high quality plywood it's a I or II.
Look on the underside of the pocket casting there should be a date inscribed into the metal. Early GC-I has cast AL pocket castings. GC-II have zinc Alloy.
Early GC-I has fixed legs where the later GC-I and beyond have adjustable legs.
GC-I has figure 8 plates for the rail bolts. GC-II has nut inserts.
There's other things as well. And remember, it's not an exact science bc if Brunswick had GC-I parts laying around and they were interchangeable with the GC-II they would use them on GC-II orders.