FQ,
Interestingly enough I was wondering the same thing several years back when I purchased a NOS S&W Chief's Special Model 60 stainless J frame that was made in 1975. I decided to call Smith and Wesson and after I posed the question the person I was speaking with said he wasn't sure but would hand me over to someone who would know. Much to my surprise and delight Roy Jinks himself came to the phone. After a very nice conversation and answering several questions about the gun and confirming date of manufacturer as 1975 via serial number, Mr. Jinks said by all means I can shoot .38 +P loads through the gun as long as that is not the sole load used constantly. He recommended practicing with standard .38 loads and finishing up with two or three cylinders of +P loads that duplicate my carry load. I have been doing that for about three years now without a single problem. As a matter of fact the Model 60 is in my front pocket right now in a Mika pocket holster. Mr. Jinks stated that +P loads didn't exist several decades ago so obviously no such designation would have been made, but the steel revolvers made by S&W could certainly handle a reasonable amount of +P loads run through them.
I would recommend exactly the same, run a couple or few cylinders of +P to finish up a range session and carry Speer Gold Dots 135 gr. Short Barrel +P loads, the NYPD load for those veteran officers still carrying revolvers. A great round for a snubbie or any other .38 revolver. It's what I always carry and also finish up range practice with, after practicing with standard .38 Spl Federal American Eagle. Should you decide to carry just .38 Spl. the Federal Nyclad round is back which is certainly great news as well. This was the NYPD load prior to Gold Dots and I still have a few boxes stashed away that I was saving mainly as a collectible. But now that Federal reintroduced Nyclads at last year's Shot show, I may just give them a go at the range.
Good luck and let us know what you decide.