Jack the four digits are the "real" serial number. The last two digit of the year are placed on the left of the pistol (i.e. 73 for 1973).
The year is not a good parameter to know the model. In fact both models were produced in parallel starting from 1973. In the picture attached the 29/70 model manual.
Ciao
http://boards.rennlist.com/lfupload/29-70.jpg