M/M = millimeters... they chambered the 1100/1200 in 6.5X55 for European market through the late '60s, '70s and until they stopped producing them in the late '80s. Your rifle was definitely made in 1977 as '83 proofmark is J - B... I don't know were you got the info it was only made in 1983 but it's sure wrong.
The 1000/1100 is the standard grade (without skip-line and with a small schnabel tip) and the 1200 is the "de Luxe" model, with skip-line checkering and rosewood fore end, plus adjustable trigger which was an option on the other models.
For '84, they introduced a lighter version of the 1100 with hollow bolt pommel and alloy trigger guard / floorplate.