Yea, the heavy model 1763 was the prototype new model musket.
It went through a series of changes, the french really wanted a heavier musket that could stand up to abuse, like a brown bess. The earlier patterns of the 1717 series were very fragile. Locks failed, stocks often broke and parts became unstable in the stock because it was so slender.
One big issue with the 1717 series was the loading rod, it was a very skinny iron loading rod and when the soldiers returned it, it would often slip past th middle band, so a rammer guide was added to 1763 heavy model, sometimes referred to as a spring too. It was 12-14” long and made of sheet steel and riveted to the front band. While it was a good ideal, it was not practical, all they had to do was angle the lip of the front band upward and the rod guides itself. They ended up deleting it around 1764/65 and made another heavy model 1763 which very much closely resembles the miroku 1766, these are very rare, one is on display at Ft. Ticonderoga.