Groo here
S&W says if the gun is steel and stamped with the model[ M10-5 etc]
Shooting +p is fine.
I think all steel 38 revolvers starting with the mode stamping were proofed with the 38-44 highspeed loads;[even snubs]
. remember most people shoot more in a year than the older guns ever had through them and light loads were used for range use....