Can't verify this as fact, but It is my understanding the MkII came out about 1981. That is around the time they changed from a plastic trigger to metal. Not sure what other differences there are except I read some MkII are stamped on the receiver.
Below was copied from http://forum.vintageairgunsgallery.com/feinwerkbau-fwb/feinwerkbau-serial-numbers/ Puts your rifle around 1982-83 making it a MkII. FWB Sport 120-series (Manufactured 1973-89) 8899 -1973 Factory verified 10012 -1977 17023 -12/78 18746 -1979 34378 -09/80 34476 -09/80 39127 -1981 40003 -05/81 44001 -09/81 49992 -10/82 51631 -1983 52589 -05/83 56026 -1984 59458 -1985