Te dejo un artículo de una persona que sabe mucho. Yo no lo sabía pero al parecer es como tú dices, a partir de 1983-1984 estos números pueden no coincidir.
BMW motorcycle identification VIN numbers
bmwmotorcycletech.info
"Near the end of 1983, with the introduction of the 1984 model year motorcycles, BMW decided to not match up frame and engine numbers, and, in fact, BMW totally eliminated the serial number stamped next to the oil dipstick. They said so officially, although BMW did produce some engines with the 7 digits stamped next to the oil level dipstick when BMW said they would no longer do that. There have been a few widely-scattered reports of differing serial numbers on the same bike. My guess is that BMW utilized some engines, frames, etc., that were in inventory during the transition period. There are not many of these. BMW also re-incorporated stamping the full VIN into the lower right frame tube. That stamping can vary with the particular Country the bike was shipped-to. There are some Countries that require special plates or stampings, and I will not be getting into those further in this article"