The built in finder mask on the Contax I. Most, like the one above, are for 135mm lenses, but they were made for 85mm lenses as well.
The mask finder for the Contax III 544/7. The small knob flips the 135mm mask out of the way, converting it to an 85mm mask. Another similar finder with a shorter arm, 543/7, was made for the Contax II, as the shoe was on the top plate rather than on top of the meter.
The finder 544/7 mounted on a Contax III with an 85/ Triotar
Operating on the same principle as the prewar mask finder, the postwar model fits around the end of the camera. Now, one size fits all.