I have been changing the levels for the original Lemmings, and I think I just found out why they suggest to use CustLemm instead. That is that I have arrived at Fun 8, which is a duplicated level, and even though I change the information in oddtable.dat by using a hex-editor to what all the "non-used duplicate"s have there. It looks something like this ".c.c.c.c.c.c.c.c.c.c.c.c This is a non-used duplicate ". The periods are actually 00 in hexadecimal. But changing the place for level 8 Fun in the oddtable only changes the level to this in the game: There are 99 Lemmings, you have to save 99 Lemmings, it's at 99 speed, you have 99 of Climbers, Floaters, Exploders, Blockers, and 20 Builders, Bashers, and Miners, and 32 Diggers... and once the 81st lemming comes on the screen Lemmings messes up, but actually I expected that to happen.
Is the actual information as to whether it's a duplicate level held in Main.dat? I guess it would be, though I don't know.