I got myself a nifty digital scale a few weeks back and ever since I;ve been experimenting and checking out various counterweights and stuff. Since most of the engine counterweights arent marked, its easy to get them confused once you take them off the engine they came on. I usually mark mine, but it'd still be nice to be able to differentiate. SO, I've been weighing them. Here's what I found. This applies to automatic rear counterweights only.

GSLSE s3 11.5 rotors, ctwt weighs 4.30lb
86-8 s4 10.5 rotors, t2 and NA, ctwt weighs 4.04lb
(I havent gotten time to weigh the s5 ctwt I have, so no numbers on that yet, though it SHOULD be the same as FD)
93-95 s6 9.5lb rotors (FD/turbo) cwtw weighs 4.03lb (not enough of a difference to distinguish from the s4)

I compared the s4 and s6 ctwt and can find no difference between them at all in weight or dimension. THe s4 was ordered brand new, and the s6 was removed from a stock original FD core motor, so I trust these results. I plan to weigh the s5 ctwt soon and see what my results are.