With a little guidance from Harry I have confirmed that the regular RF-coupled 100/2 lens (at least the one I have) does have a removable head.
I had owned this lens for years and had never guessed that the head might come out -- despite having owned a first-series 90/2 Summicron which had a removable head that worked on exactly the same principle.
The 100/2's head is retained in the focusing mount by a fine-pitch thread. Thanks to the fineness of pitch, the head fits quite securely into the mount without requiring a massive amount of tightening torque -- it's unlikely to come unscrewed by itself, which is probably why I had never noticed that it's removable.
Yet, once you've overcome the initial resistance, it unscrews quite easily. The only slightly fiddly aspect of the operation is that the thread is so fine that there's a risk of cross-threading it on reinstallation.
Even though I don't have any of the focusing adapters, or a Mirror Box 2, I discovered one additional use for this capability: It makes it much easier to clean dust off the rear element, which otherwise is difficult because it sits so deep in the rear tube! I had always thought my 100/2 was pretty clean until I removed the head... then I was SHOCKED to see how much dust was on it!
Am attaching some quickie piccies for those interested in seeing how it goes together.
Incidentally, all those rumors you might have heard about this lens' supposedly fantastic performance... well, my opinion is that they're absolutely true. I'm not necessarily saying it's hugely better than the legendary 105/2.5 Nikkor, but it's at least as good, and it's a half-stop faster. Almost makes me wish I had one of those adapters so I could wangle it onto an SLR occasionally... it yields great results on my R-D 1, but having to use an accessory viewfinder for it is a nuisance!
