Focomats, Focotars, and extension tubes
I have a couple of Leitz Focomat Ic's (and a box of parts from a DOA third enlarger).
One of these came fitted with a 1952 vintage Focomat (Leitz P/N DOOCQ) with a firmly attached (via a setscrew) DOORX extension tube. The lens board on which this Focotar mounts has a substantial recess from the front edge of the helicoid tube.
The second Focomat Ic came fitted with only a 1957 vintage DOOCQ Focomat (no extension tube). The focus cylinder in which the lens mounts has a smaller recess from the front edge of the tube. Clearly, Leitz seems to have eliminated the need for the DOORX extension tube by moving the lens mount further from the tube. This Focotar has an asterisk at the end of its serial number. I calibrated and used this Focomat for years with no problems.
I just fitted an El-Nikkor 50mm/2.8 to the second Focomat (the one with the shallow recess) and, after a modest bit of time setting the thing up properly, now works properly with the Focomat autofocus at f2.8.
The scrapped Focomat the provided my parts collection has the more deeply recessed lens board. It came fitted with a camera taking (collapsible) Elmar 50mm/3.5 lens, fitted with the extension f-stop ring.
I ask the forum why Leitz used the extension tube in the first place - is anyone familiar with this? I invite comments on this bit of Leitz arcana.