The best we have is TractorData.com - information on all makes and models of tractors which shows that the Snapper models were built between 1979 and 1984. The Massey Ferguson models predated the Snappers by 2 years (1977 to 1979 or '80. All were built by Snapper and were basically the same tractors in two different wheelbases The 1650 was 5" shorter than the 1855 and the MF models had the Sundstrand hydros as opposed to the Snapper models with Eaton 11 hydros. Later production models of the MF1450 and 1650 had the same Eaton hydro as all of the Snapper models.
Both models (and makes) are very strong pullers with the 2-speed final drives and have very robust frames.
While your tractors have the Snapper logo on them, there have been more threads on these tractors in the Massey Ferguson sub forum than in the Snapper sub forum. All have the same model numbers, but there were a couple of extra MF models. I have a pair of MF1655s The first two digits state the original horsepower, the third digit indicates a hydro transmission, and the fourth either a single cylinder Kohler engine (0), or a twin cylinder Onan engine (5). The Onan, being longer than the Kohler, required the stretched frame which also allowed for a more robust front PTO system.
Sorry that I can't pin the manufactured date down to something more precise. We used to have the serial number by year info for the MF tractors, but the data got lost when we changed the forum platform a few years ago.