Why the unthreaded socket on the R8 base?

On a mailing list in June 2000 "Gerry Walden" <gerrywalden@cwcom.net> noted the following:

When I got my R8 I asked what the unthreaded hole in front of the tripod mounting bush was for, and was told that it might be for a couple of accessories. The hole is replicated on the motor drive. I have now found one of those accessories. I acquired a Gitzo tripod (G1201) with a ball and socket head (G1276). The head takes an oblong camera mounting plate which has a spring loaded stud which fits neatly into the hole and stops the possibility of the camera rotating on the head. This makes this tripod (and presumably other Gitzo tripods) a very neat, well made and sturdy tripod for the R8 in particular.