Everything that I have been reading states the the nv3500 has evolved from the hm290 which came out in 1988. Then the hm290 was reengineered and renamed the 5lm60 in 1990. In 1993 the 5lm60 was gone through and renamed the nv3500. Externally all three tranny ies are the same but that is as far as is goes. Internally the trannies use different parts from each other. The only way to tell the nv3500 and the other 2 apart is the nv3500 has a single shift rail design, where as the hm290 and the 5lm60 have the cumbersome 4 rail design. There are some other differences but that is the easiest way to tell them apart. As for the new(99-up) style tranny you cannot use it in the gen I or gen II blocks without an adapter kit it is the same if you were trying to mate a late t56 to an early style block.