Mamiya manufactured a number of variants of a medium format folding camera, the Mamiya Six in the 1940s and 1950s. Many of them have a nut standing proud from the base that can receive a tripod screw.

Mine is a model P and given the surface area of that nut, there's a lot of wobble when it's on a tripod. This plate stabilises that wobble.

For convenience I fixed it to the base of the camera with double sided tape.

Looking at images on ebay, it should fit the following models:

Model IV B
Model V
Model K
Model P