12/77 on year of transmission but engine don't match that year. In march 1978 was start of 4 speed. More than likely you serial number something like 120560303849 that put date around 11/92 that year 7hp kohler was used and be 4 speed with pto model. This info comes from yahoo group site you can join and down load files there for transmission. MTD site has section for troy bilt tiller down load manuals you need.