Sorry, I cannot tell you the timestamp right now. I have a CD version which contains both Original and More Lemmings, but I couldn't find it (maybe I can look it up in my archived CDs within the next days). The files on my PC are from a backup CD I created several years ago, so the timestamps show some date in 1999, which is hardly the correct one. I don't even remember if I used the CD-version for the backup or if I downloaded a copy from the net this time... If it helps, the CRC-32 of the executeable is 61459C25.
I think there are several things in the original PC version of the game that were not actually planned this way. Mayhem 4 with only 80% to save is next to pointless, you can even achieve this by entering the web from the upper side, rendering the idea of the level, preventing each and every lemming from turning, useless. Same goes, for example, for Taxing 12. The idea is not bad at all, but with more builders than really needed, it's too easy. Another example would be Wicked 14, which is surprisingly simple compared to the levels before. I will also never understand what made the developers of the PC version exchange "Going their seperate ways" with "All the 6's" (Tricky 21) ...
Well, to cut it short, I think it's a real good idea to have the ability to override levels and properties. Maybe Eric can, despite the goal to clone as exactly as possible, include some of the often more logical Amiga version stuff by default (especially restoring the original Tricky 21). If he is willing to do this, we could gather a list of the most obvious PC mismatches.
Apart from that, having the ability to override whole leves would also simplify the use of custom levels with LemmixPlayer.