I'm going to toss this in here since we were talking a bit about it and because I have screenshots. Hopefully this is interesting to someone.
I've made a copy of the 12-level Mac demo I have, called the "Lemmings Mini-Game". Apparently other demos are only 4 levels. This one came on the Macintosh Productivity and Entertainment sampler CD packaged with a Macintosh Performa 600, purchased in 1993. It contains one interesting level I've never heard of anywhere else: Fun 2 is "The Apple Computer Level" which is winnable without doing anything at all. Fun stuff. I wonder if it's implemented the way the "special" levels are, or as a normal tileset.
The level list is as followed (except Mayhem, because I ragequit after screwing up The Boiler Room's solution three times in a row)
Fun 1: Just dig!
Fun 2: The Apple Computer Level
Fun 3: Now use miners and climbers
Tricky 1: This should be a doddle!
Tricky 2: Lemmings in the attic
Tricky 3: Ozone-friendly Lemmings
Taxing 1: Watch out, there's traps about
Taxing 2: Come on over to my place
Taxing 3: How do I dig up the way?
Mayhem 1: The Boiler Room
Mayhem 2: ?
Mayhem 3: ?
I've grabbed a few screens from my Power Macintosh 6100/60 (would've used the Performa but the power supply needs replaced before it'll boot again). The sound doesn't work on PPCs, because this is from before the processor transition (they fixed it in later versions of the full game, but not this obviously).
Download (Stuffit SIT file):
here