It turned out CustLemm has a higher safe fall distance compared with Lemmings, so that for example, if you play "We All Fall Down" on CustLemm, you don't need to dig at all.
Since a few people here designed and tested their levels with Lemmings rather than CustLemm, I once made a modified version of CustLemm based on Mike's info to change the falling distance to be same as Lemmings.
Since most levels do assume CustLemm is used, for the most part you don't need the hack version.
There is also an unrelated hack that allows the original game music to be played. A0;CustLemm currently will play music from a CD, but if you don't put a CD in the CD-ROM drive, you get no music at all.
All these happened months ago but I know Mindless's web pages has them available for download, but I don't remember offhand what the URL is. A0;Try searching the forum with keywords "custlem2" and/or "nocdlem".