FWIW, my 1981 is SN 6385209, manufactured 10/1980. It has twin ATEs. Having said that, I found on the net a fiche printout (from RealOEM?) listing a twin-disk retrofit kit, which would make a reasonable person consider the possibility that BMW might have made a mid-year change in the 1981s.
There is precedent for this with German manufacturers. I have a '66 VW that originally had a 6-volt electrical system. I have read that VW changed over to 12-volt systems *during* 1966 production, making the VIN of a 1966 particularly important.