The pic above shows a very original 1931 hinge mounted LH mirror arm as used on pickup trucks and commercial vehicles. The original arm was a forged steel arm. This is the late styled arm which is at the top level of the hinge.
Earlier 1930-31 hinge mounted mirror arms were a different style which placed the arm at the center of the hinge.
This late arm is the basis for the many reproduction arms made over the years in both chrome plated brass and chrome plated zinc.
The pic above shows a comparison of several old chromed brass reproductions, as well as an original painted steel arm and mirror head.
The black arm and head are early styled originals, with a single hinge screw hole in the arm and the three screws at the mirror head pivot ball.
Below the black arm is an older chromed brass repro with a single arm hole and a head similar to the original.
The two mirrors and arms on the right both have two arm holes for the screw to the hinge. Note however that the hole position relative to the hinge center is quite different between the two reproductions! The arm pictured in the upper 'right' is clearly "wrong"!
The mirror head on the upper right has no exposed screws whereas the head at the lower right has three big screws. Most repros are being provided with Phillips head screws, though this old one has slotted head screws.
The pic above is of a very original (repainted) late 1931 Budd steel cab with the styled steel mirror arm, and clearly showing the original attachment of the arm to the hinge with two pan head screws.
There are two different versions of attachment of the original arms. The early arms had only a single pan head machine screw drilled and tapped into the hinge, whereas later versions used two pan head screws per arm.
Reproduction arms are now supplied with set screws as part of the arm, but this is entirely wrong. The original arm was clamped to the hinge with the screw, not pushing against the hinge with a set screw. The set screw cannot do a proper job to affix the arm securely to the door!
For comparison, an original single hole steel arm is shown with a chromed reproduction above.
Pictured above is a close-up view of a quality chromed brass reproduction arm made in the 1970's. Note the sand casting marks in the unfinished area. The show surface was ground smooth and plated.
This example is the best fitting reproduction I have ever been able to find. Modern reproductions all seem to be ill fitting zinc diecast construction.
The last pic shown above is a close-up of the pivot ball area on some reproduction mirror heads. This design limits the amount of angle adjustment of the head and sometimes cannot be adjusted to the needed position, or retain the position well.