Yes, 675 would be correct for the first part of the model/serial number.
675 - is the Yamaha designation for the model this is a "carbon copy" of - in this case, a Yamaha 60 AE which was mfg between 1977-79
L - long shaft (20") - built for a 20" transom
322779 - the serial part indicating it was produced in 1981 (as you thought).
These were built by Yamaha for Merc as a lower cost alternative to the main Merc line.
While these were good quality what this 1981 model was, was simply a (reissued) 1977-79 Yamaha - so not "leading edge" when it was new which meant it could be sold at somewhat of a discount - think of it like buying a "non-current" motor.
The 55/60 horse twin cylinder Yami/Mariners were never overly prolific so sometimes finding parts to keep them running can be a bit of a challenge.
They were far more popular in Australia and parts of Asia than North America, so depending on where you are, used parts might be common to almost "never seen".