The turret finder has a variety of optics, each framing a different focal length. The front of the finder is rotated to bring the desired focal length to the eyepiece. The very earliest version lacks click stops.
A very early turret finder with optics covering 28, 50, 85, 135, and 180mm lenses. It was catalog number 436/4
A very later finder, catalog number 440. It covers 21, 35, 50, 85, and 135mm lenses, but can also be found with a 25mm optic instead of 21mm.