FWIW, S/N 41,9XX is one of 5,600 Astra 400's sent to the Madrid Armory in 1931. That was over 75 years ago.
Because of the age factor, most of the recoil springs in these old Astra 400's are shot. Spend $10 and get a new one from Wolff. Since the 400 is blowback and the recoil spring plays a big part in controlling the recoil, that's the least you could do to protect yourself and the gun.
Sarco is sitting on a large cache of non-corrosive Spanish Santa Barbara 9mm Largo ammo for $170 per thousand. All that I have seen has 1970's dates.
If the chamber is marked 9mm/.38 then the gun has been modified to accept .38 auto ammo, none of which is readily available. However, you can load .38 Super cases down to .38 auto specs and come up with cheap ammo that won't beat the gun up. Be advised, the Spanish 9mm Largo ammo is pretty warm.
-Lee