And how do you tell them apart. I have 5 of them in a crate and need to put 1 on a Gen 6 and save 1 for a MK4 and don't need 3 sitting around...Thanks.

From what i know Yes, it's different.
They have the same bore, same shaft diameter, same counterweight.
The difference is in the depth of the hub. The Vortec (Gen6) version is shorter by exactly the thickness of the reluctor ring. An aftermarket weighted balancer can be used if the hub is machined down to the correct depth.....i have no first hand experience with this just what i remember reading about so hopefully someone can confirm this...

So that would be machined off the back side? Reluctor under timing cover? So a Gen 5 10 bolt cover would have Mk4 balancer? All 3 gen6 I had, I never saw a reluctor. 1 was industrial engine with 4bbl manifold. One from 98 suburban-tahoe, one crate 461 hp 502.

like i said i have no first hand experience but yes off the back side...if there's a reluctor ring then the hub would be shorter another thing i think may be different is the center opening....the gen4 pulleys i believe have a bigger lip on the back side (the side that bolts against the balancer) the gen6 stuff i think has a smaller opening and a smaller lip.......again i haven't played with this stuff in a good two years so memory could be off on this....why not call mesa balancing and ask them

I know the Mark IV and the gen V are different. They will both fit on the crank but the weight is different.