Possibly some different alloy used as well, given the later M-40 release. It was all so hush hush about it in those days.
Dennis Young Hotaka, Japan
Dennis referred to the G-40 to MA-40 change as a relabeling. This is not really correct. The MA-40 did replace the G-40, but they were slightly different rims. Both were hard anodized clinchers of the same approximate dimensions, but the cross sections were a little different. The G-40 was more rounded and looked a lot like a tubular rim. The MA-40 was really boxy. The first MA-40 I saw was in the fall of 1985, but I suspect they had been around for at least several months by then.
Kim, you may be thinking of the GP-4 or GL-330 or the GEL-280, all of which were tubular.