The original Basic Roleplaying (1980, I believe) was 20 pages long. It's no longer in print, but you can occasionally find copies of it on eBay for a reasonable price.
SPQR is definitely a very likely candidate. Perhaps HR. That said, I now know for certain that four of the five systems were OD&D, AD&D, Arduin, and Runequest.
Oh, I got the big one (softcover). Title page has a copyright date of 2008. I know that there was also a hardcover, but I wasn't sure which came first.
I'm trying to find as much information as I can about Steve Perrin's North Coast campaign. He mentioned it in an interview, where he said it had been played under at least five systems and I personally know that D&D and Runequest account for two of those systems. I don't know much else (yet). Any information you can provide would be much appreciated!