It is well made, works perfect with Auto Bellows Rokkor 100mmF4, and
the base mount (camera side) is easily replaceable for the other mount.
Bellows is extendable by hands, round nob is to hold the position of
the bellows, and also you can get M42->SR or L39->SR adapter easily.

(Actually I converted it to E-mount one for Sony ILCE-xxx cameras.)
was $56; mount to mount thickness min = 40.4mm, max = 99.1mm FYI

However it is not easy to focus using this bellows, you still need lens'
focusing mechanism and/or third party helicoid for easy focusing IMO.