Isco-Gottingen Tele-Westanar 1:3.5/135 question

[blumoon]

I have this Isco-Gottingen Tele-Westanar 1:3.5/135mm with M42 screw mount.
The lens is in good shape with clean glass.
My question is: what is supposed to go in the empty hole with the threads? Shown in first pic.
I've been using the lens on a Pentax K3, and a Lumix G-7 with decent results.
Took this pic of Augie today with lens on Pentax K-3

 

[Mos6502]

That's where the cable release socket should be, but somebody has removed it. If you're wondering why the lens would need a cable release, it was intended to preserve the automatic diaphragm operation when the lens was put on a macro bellows. You would use a double cable release, one cable to operate the camera shutter, the other to stop down the lens diaphragm.

 

[blumoon]

Thanks for your reply