M to R adapter? What is this?

[pgs@thillana.lcs.mit.edu (Patrick Sobalvarro)]

I saw something rather strange at the San Mateo show last week, and
I've only just remembered that I wanted to ask people on this list
about it.  A dealer whose name I cannot remember showed me an adapter,
with no glass at all in it, just a simple tube about half an inch
long, with a female M lens mount on one end and a male R mount on the
other.  I am sure about the sexes of the mounts.  This device was made
by Leitz, and had a catalogue number on it which I unfortunately
forgot to write down.

But this would seem to be a useless device, because of the difference
in registration of the two cameras.  You shouldn't be able to mount an
M lens on an R body and maintain infinity focus without using some
negative optical elements in between.  In fact, given its length, one
would expect that this "converter" would in fact act as an extension
tube of some sort.  So what is this thing for?

-Patrick Sobalvarro

P.S. I got the R6 body I ordered on time and apparently in working
     order from Don Chatterton.  It cost only $950, because the
     previous owner's carrying strap had scratched a fair deal of
     paint off the (black) body.  But parts like the lens mount,
     winding lever, film rails, and shutter-speed dial show little
     evidence of extensive use.  I'll know more when I run some film
     through it this weekend.