Summicron c 40mm adapter ring

Leica did a dirty on us, they threaded this lens with a extremely rare 39mm thread, with a pitch of .70mm, the standard 39mm filters have pitches of (.50mm E39) and .75mm (more modern thread for most Japanese cameras) 

Our adapter ring screws into the 40mm lens and then accepts a E39mm filter, meaning you can use your standard filters with this lens.

The ring is solid brass with black paint and is a total of 6.32mm in length, one screwed into the lens it will add 3.12mm to the lens plus your filter.