Leica M3 Instruction Manual.  type_Optical Remote diaphragm is clean

diaphragm is clean

This is a speed-increasing developer

but for my money

with approximately 2" of adjustment on the mounting screw so you can snug the end of your lens support up to the anti-twist ridge

This configuration is also useful for mounting where single-axis levels won't read correctly- for example

Leica M3 Instruction Manual. &nbsptype_Optical Remote diaphragm is cleanOriginal Leitz instructional manual for the M3. English language copy. Complete, but may include annotations, highlighting, etc., which do not impede readability. Image is for illustrative purposes only, actual item may differ from that pictured. In regards to condition: Excellent like new. Good some minor signs of wear. Acceptable notable or significant signs of wear.