you're right Art. Take a look at the top of the receiver (found on the internet...mine looks the same)
[url="https://www.milsurps.com/asset.php?
Attachment 113023
It appears to have started life as an a3 at forging time, but was then finish milled to an m1903. Best hypothesis on these is that they kept making m1903 either to use up their stock of parts, or whilst waiting on the a3 specific parts to become fully available. I read somewhere that there was delays, for example, in tooling up for the a3 rear sight. I can only wonder if the SA replacement barrel 5-42 was original. Likely not with the short chamber issues I'm describing.