I am writing the second part of my blog post on man-pages and I am trying to get to the bottom of the origin of the MANPAGER variable. It seems like it may have appeared as a modification to the 4.3BSD man command in NeXTSTEP some time between 1987 and 1989. It didn't have PAGER because the import of 4.3BSD-Tahoe to the code base didn't happen until 1989.
Does anybody have any firsthand knowledge about this? Did it maybe show up somewhere else before NeXTSTEP?