This gentleman should take care about the ammo he feeds that revolver. It won’t take any +P loads at all. And I seem to recall that it fired a lower powered load that was specially made for the military. It was an FMJ if I recall correctly.
I’ll have to do some research on it later.
It should be safe with any modern standard pressure .38 Special load. Smith heat treated the cylinders of guns of this period. The FMJ is probably in regards to types of bullets permissible under the Geneva Convention.
I recall a few years ago there was much discussion of the use of +P in pre-war M&P’s on the S&W Forum. Some individuals with old load data concluded that +P (post-1974 IIRC designated ammo) was equivalent to standard loadings of the time. One member ran ~500 +P rounds through a 30’s vintage M&P with no discernible ill effects.
Not saying that it should be an accepted practice, but carrying with +P should be OK if the gun is in good condition.