Capt. Chas. Askins wrote in one of his books that the hammerless sidelock gun was just an evolutionary step from the hammer gun, most of which were sidelocks, to the hammerless boxlock.

Some U.S sidelock hammer guns --

Baker Model 1897
Colt
Crescent
L.C. Smith
Meriden Fire Arms Co.
Parry
Parker Bros.
E. Remington & Sons
Remington Arms Co. (Probably the most common U.S. hammergun)
Sneider
Wilkes Barre (follow on of the Parry)