Similar to my Mk7 on my Carlton Constellation.
Link to my web page:
These are typical of the Cyclo Benelux 'dancing chain' type rear mechs.
My Carlton dates from 1958, so your mech is defo around this period.
These mechs were specifically designed to accommodate 3, 4 or 5 sprockets. Since they can only be adjusted in one direction, as the large spring which pushes the cage inwards as opposed to a parallelogram mech which pushes out/away from the largest sprocket, is a fixed length. With careful setting up these work well. The main adjustment is achieved by rotating the large knurled nut through which the actuation toggle chain passes.
This link should help too!
Hope you use it!