I [sic] have to look into my catalogues to make sure but here is the general breakdown of C40-C50:
1993: C40 no B-stay, no Ti BB shell, no HP chain-stays, steel forks
~1995: C40 with B-stay, Ti BB shell, no HP chain-stays
~2002: C40 with B-stay, Ti BB shell, HP chain-stays
2003: C50 with B-stay, Ti BB shell, HP chain-stays, 1-1/8 head tube, bigger top/down tubes
~2005: C50 with B-stay, Ti BB shell, HP chain-stays, 1-1/8 head tube, HM CF tubes, shorter lugs
So as others have said it's most likely a C50 that has had the HP chainstays replaced at some point (or built without them).