Long ago Colt made a round they called 38 ACP - 38 Automatic Colt Pistol. Then they put more powder in the same dang case and called it the 38 Super Automatic. They chambered it in the 1911, which was strong enough for it, but it will fit in the older 1908 38 ACP. Really, REALLY stupid thing to do.
38 ACP would be the 38 Auto. Same size as 38 Super, but about half the power.
It would be safe to fire it, but it probably would not work the action. Not enough "umph" to push the spring back.
Also, since 38 ACP has been an obsolete cartridge for years (like many old rounds, they quit making them for WW2, and never started back after the war), it would be in the realm of "collector ammo", and undoubtedly worth a bit of money. Why shoot 5-dollar a round pistol ammo, when you could sell it and buy 50-cent a round ammo.
The Spanish Model 400 (Modelo 1921) is marked "9mm (38)"
It's not designed to shoot the 9mm luger or the 38 Super, but there are people that will give you an argument on both counts. It was chambered for the 9mm Largo and it's a good idea to stick to the cartridge any pistol is chambered for.