sl has ti, chromo or manganese rails (maybe vanadium too...) and a shovel nose.
I believe (this may be completely wrong as i'm going from a hazy memory as never had one as the sl is the BEST EVER) has a round nose and the rails are suspended from the shell with rubber inserts (hence the tt trishock moniker).
also the sl has the plastic tab at the back that goes across the back, the tt doesn't
vetta sl's come up on ebay fairly regualarly (so my saved searchs keep emailing me) but the ti version less often. Picked up a NOS on last year. the non ti ones come up all the time and are cheap, the tt seem to come up all over the place, but don't know what sort of rails...
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