My extension tube set (Soligor for Pentax AF + P/KA) was not working correctly on my DSLR, so I checked the tubes, and found an interesting difference. It has three tubes (12mm, 20mm, 36mm).
This is a photo of the 20mm tube:
and this a photo of the 12mm tube (the 36mm is similar to this one):
Note the difference of the third (from the left) pin. On the 20mm tube it is similar to the other pins, on the 12mm tube it is always recessed.
According to Bojidar Dimitrov's K-mount page this pin transfers the "A" aperture settings information:
On the body mount, the "*" contact is slightly recessed. On the lens mount, "*" is a small pin whose head can be raised or lowered depending on the position of the lens aperture ring. When a numeric f-stop is selected, the pin's head is lowered so that it does not make contact with body's "*" contact; when the "A" aperture setting is engaged, the pin's head is raised, and it makes electrical contact with the body's "*" contact.
This explains the strange behavior of the extension tube. If I use the 20mm tube the camera always thinks that the aperture ring of the lens ( SMC PENTAX-A 50mm f2 in my test ) is set to "A" setting. It also explains why my camera+12mm+20mm+lens setup was not working the same way as my camera+20mm+12mm+lens setup.
Is it some kind of bug (they mixed up the tubes)? Or maybe it's just an undocumented feature of the extension tube set to have different elements? Is there a camera+lens combination when the non-recessed pin of 20mm has advantages?