A Little More Information
MR=December 1980 is correct. That was the month that the frame was welded.
If you look on the headbadge, you will find a four digit number. This is the day that the bike was assembled. The last digit for the number on your headbadge is most likely a "1" as the bike was probably assembled in 1981. The first three digits are the specific day of the year. For example, the number 032 would indicate February 01.
We currently own a '69 Twinn, '77 Deluxe Twinn, & a '79 Deluxe Twinn.