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How many cylinders does this have?
There was never a 50 horse Yamaha/Mariner, so you must have one of the Merc/Mariner models.
The 1983 model was a 4 cylinder but your bit of serial number tends to indicate a 1986 3 cylinder model (unless that B is actually a 6 - 613 would be a start serial for one built in 1982).
Looking at the parts lists, neither had a thermostat (not uncommon on older models) but the 3 cylinder did have a temp sensor on the waterjacket cover (neither model had a "head" in the traditional sense).