Jim, the information on the subject I've read say that the Astra 400's that are marked 9MM/38 were intended to be shot with either the 9MM Largo or the .38 Auto, and if you have a gun so marked you will see that the rim area of the chamber is bored a bit larger to accept the .38 Auto's larger rim. The .38 Auto is very similar beyond that, and the loading is less powerful.
The proof (If I'm correct) is that many 9MM Largo chambered pistols won't chamber .38 auto/.38 Super (Same case dimensions) because they have the rim interference, but those pistols aren't double stamped.
My Star Model A is stamped for both cartridges as is my Astra 400 (Model 1921) and in fact Stoeger ads from the 1930 sold these pistols
for .38 automatic cartridges. I may be wrong, but I believe these were marked 9MM/38.
The 9MM Largo website (
http://9mmlargo.com/) Talking about the model 400; "Some pistol barrels are marked '9mm/38' indicating pistols that were made with/converted to a compromise bolt face that will accept either the 9mm Largo cartridge or the .38 ACP (not Super) cartridge.". (Note that the mention was of the "Bolt face", not the chamber mouth, though it's clear that the chamber mouth on my model 400 has been enlarged.)
In the same website, speaking of the Star A model; "To further confuse the issue, as pistols were returned to Star by the Spanish Government as surplus they would be refurbished and if necessary a new serial number applied. It was at this time that the mystery 9mm/.38 markings were applied to the pistols if they were reworked to handle the semi-rimmed .38ACP cartridge."
There's no doubt in my mind that pistols marked "9MM/38" were intended to be shot with both 9MM Largo and .38 automatic cartridges, and was told in a gun shop in Yuma that .38 Automatic cartridges are still available. (?)