This is a real Sidelock
how it looks inside:
Sidelock Guns usually are to be find on better and more expensive guns. Indeed the sidelock has some advantages and looks quite nice, especially with some engravers art. But one should be careful with nice looking Sidelock Guns, because sometimes it is not, what it seems to be.
Looks right, but isn’t! This belgian sidelock is a fake! Look behind: