Here's another level to add to the Extra Player, which comes from the Amiga Lemmings demo (http://www.lemmingsforums.net/index.php?topic=1803.msg47381#msg47381).
Look for the file "Level001-2.lvl". The level is called Battle at Midway, which I don't think appears in any of the 2P levels of Orig Lemmings. As this is the only unique, it should go in "Other".
From the other topic, there's currently a consideration of moving the 2P levels from Orig / OhNo back into Extra. If this were to happen, I'd think it better fits with them than with "Other", even if it comes from a demo.
But it absolutely should be added. Thanks for digging this up.
(I'm looking through all the levels rather than just that one; I notice some traps are missing too. For example, Taxing 1 doesn't have one, Taxing 2 has one less than normal...)