Do you know which engine you have? Is it a K3-A or K3-B? The K3-A was in the Mitsubishi 373 also known as Beaver III. The B engine was in the M 470 Buck. The Buck engine has 18.5 engine hp. The Beaver has 15 or 16 I think. Otherwise both tractors have 3 speed with high and low range and 3 speed pto. There is a dealer with a website that has a link to some data that may help you. satohparts.com is the place. Edit: I just looked it up. It's a K3-B making yours most similar to the 470 Buck. I have a Buck and the gray version of the Beaver III. Good tractors.
It's actually closer to the MT160-180 which succeeded the early 80's Beaver and Buck. While they used the K3A and K3C engines, the frames, transmission, sheetmetal etc. are closer to yours than the older Beaver and Buck.