I just got off the phone with Mamiya USA and I've been told that the first letter of the serial number is coded for the year the item was made.
For example, I was told A=2001 , B=2002 , C=2003 and that P=2000.

But I did not catch the rest and feel a bit funny about calling back.

So can someone tell me the rest of the code? Specifically what O Q R S mean.