The English made guns I have, an assortment of pistols, shot guns, and rifles of various grades from government contract cheap to fairly high grade, nothing a King or Earl would own though, and the hundreds of ones I have looked at photos of, all have the screws slots oriented in some logical fashion. The lock plate side screws had dealer's choice for horizontal or vertical, but the do stop at one or the other. The barrel tang, guard, and butt plate screw slots always line up with the long axis of the gun on all of them. I have noted that the slot orientation will vary due to stock shrinkage and over tightening. Even when they don't at the present time, the original intent is obvious because of the shape of the head and how it fits into the mating part. The hammer screws line up with design lines from the lock layout. Some being horizontal or vertical when the hammer is at half cock generally, sometimes when the hammer is down, or rarely when at full cock.
I haven't seen or studied enough original American made guns to have observations about them, but I suspect they follow the same trend. Getting the slots to line up is fairly easy to do when the screw heads are finish filed after inleting the hardware.
Sometimes even the internal screws slots on the locks were aligned, but this is on the more expensive guns.
Mike