"Of course there is the possibility that this might be a post-war frame drilled for a police order and then plugged when someone changed their mind, as DWalt seems to suggest..."

I wasn't suggesting that, but it is possible. What I would suggest is that even though the frames are of early postwar manufacture, S&W continued to drill swivel holes in the butt of every frame as a standard practice for a brief period, possibly in the expectation of many large police orders at that time, and so that practice would have been logical and justifiable. If they received orders for civilian non-LE sale, they simply pinned plugs into the holes. I would say that it is very unlikely that any frames of the S-series were made before the end of the war. S&W typically serial numbered frames during frame manufacture for future assembly, and the S-series frames were then necessarily of postwar manufacture.