Connecting rods sorting

The connecting rods come as JJ, II, I, H, etc. The compatibility back and forth can be quite confusing. We talked about such markings and it was suggested that it's a mark simply indicating the forge where it was made. That might be so, but I think that's not the whole picture.

Example - 1981 A3 diagram shows three different connecting rods: ROD-ASSY-CONNECTING | in.Gin. MARK 13251-1051-JJ, CONNECTING ROD ASSY,I | in.Iin. MARK 13251-1006-II, and CONNECTING ROD ASSY,H | in.Hin. MARK 13251-1006-HH.

Example - 1985 ZN shows three different connecting rods: all 13252-1051. Two marked JJ, one marked II. Than, there is the A,B, no mark. So we have 13252A, 13252B, and 13252 and 1051-JJ and 1051-II.

So.. how do you know which is which? What's the meaning of 'in.Gin' or 'in.Iin.'?
Some of them show backward compatible, some don't. How do you match them??

Cheers,
Stan