Hi,
I like Bill and James's comments about screw slots and perfection. Let me add, that I have an original London-made fowling gun from the 1760s-1770s and the screw slot for the front lock bolt (side nail in British parlance) is vertical rather than lined up horizontally with the rear bolt. It has to be oriented that way because it allows the groove filed more than 1/3 of the way through the forward bolt to line up with the ramrod hole. Obviously, "perfectly" planned.
dave