For the Cat6K, it's 5.1(1)CSX (for the Sup 1a and 2). If you have a 6513, you need 6.2(2). For the Cat4K, it's 4.4(1).

Really, though, given that we don't test such old code, you'd really be better off if you were running relatively new releases so that there is decent support for modern MIBs such as the ENTITY-MIB.