At that time, the pony would have been on the left rear of the slide, not in the center. Those were pretty lightly stamped (hand stamped using a jig) and the marking was probably removed in the polishing when the gun was reblued. (It was heavily polished and reblued; there is no doubt about that.)
Note that the gun is a Government Model, a commercial gun, not a military contract pistol, so researchers will have to be sure to get the right data for comparison.
Just FWIW and FYI (don't you just love those initials?) on the reblue. Look at the grips. They have not been refinished and show the obvious use and wear of the gun before it was polished and reblued. Also, the gun shows the rounded corners, dished markings and blurred lines almost always seen on reblued guns, especially those that were heavily rusted prior to being polished. Anyone wanting to buy a gun like that and is suspicious should ask to remove the grips. Polishers very rarely bother to work on the areas under the grips or inside the magazine well, and quite often signs of deep pitting will be in evidence in those areas.
Jim