Harpers Ferry pistol lock dated 1807 is 4-3/4" long
Harpers Ferry half-stock rifle lock dated 1818 is 5-1/4" long
Harpers Ferry Model 1795 dated 1812 musket, lock plate 6-7/16" long.
Bear in mind that:
Harpers Ferry guns were skillfully made by hand, interchangable parts not being in the picture then. I have no knowledge of dimensional tolerances on plate lengths.
Both that rifle and I appear to have gained weight over the years. This is reflected in my girth, though my overall length has not increased. Can't say about the gun lock.