Model year of a car is an American invention. The German system was vehicle model, ie 220Sb... I think it was the start of the /8's that MB mimiced the US system. Under the US system, MB model year cycles ran from what I can gather, July to December, January to June.. So your 9/72 manufactured car is a US 1973 model while a 6/72 would be a 1972 model. On the early US model Heckflossen that I had or looked at, the data plate on the radiator support has a box "Year of Manufacture" and the year is stamped in it..
In Germany, more weight is placed on date of manuacture and date of first registration.

Try contacting MB Classics division with your VIN, they should be able to tell you exactly when your car was made...