Build Dates

[Beans]

Does anyone have definitive knowledge of how to establish actual build dates. Audi 80 B2. I have a VIN commencing 81A. I have been told the A is early maker identity VIN for Audi. Would the 81 (reasonably) indicate year build. Did they also give a month or just the year? 1981 is the registration year. Some indications are a "build sheet" apparently found in varying location will provide this date. I was pondering that if, by luck, it was a very late 1980 build, I am tax free for next year rather than 2022.  Anything appreciated