Webley, what do you say to firing .38 +P+ in the S&W Heavy Duty, one which was built in 1956?
It is doubtful that S&W would give the green light due to the lack of SAAMI standard for +P+ ammunition. Because +P+ ammunition can be basically whatever the person loading it sees fit, S&W has no way of knowing that Billy Bob's "boutique" .38 +P+ loads don't exceed .357 Magnum pressures.
That being said, if we're talking about .38 +P+ ammo loaded by a reputable manufacturer like Federal or Winchester, I wouldn't be afraid to shoot it in a post-war S&W N-Frame that's in good condition (I'm assuming yours is).
I very highly doubt, almost the the point of certainty, that any .38 +P+ ammo loaded by reputable manufacturers generates more than 35,000 psi because, if it did, then it would be unsafe to fire even in .357 Magnum guns. While the manufacturers don't generally make such information widely available, the pressure specs that I've usually seen quoted for Winchester and Federal .38 +P+ ammo are around 25,000 psi which, while certainly well above .38 +P, are nowhere near that of .357 Magnum.
Also, you must understand that the .38/44 loads of decades past came very near to or even meet the .357 Magnum loads of today. For decades a popular .38/44 load was Kieth's recipe of his Lyman #358429 bullet (170-175gr depending on the alloy its cast from) over 13.5gr of Hercules (now Alliant) 2400. Many notable people including Elmer Kieth, Skeeter Skelton, and John Taffin have used this load extensively in .38/44 heavy duty revolvers and Taffin has even admitted to sparing use in S&W J-Frames (he's a much braver man than I
).
Now, my
Lyman 48th edition Reloading Handbook also lists 13.5gr 2400 with the #358429 bullet, but as a .357 Magnum
Maximum load. Now, I'm not suggesting that you go out and try to shoot .357 Magnum max loads in your revolver, rather I'm using this data as a point of reference. If revolvers such as yours are well known for being able to handle what is today considered to be a max .357 Magnum load, I very highly doubt that commercial .38 Spl +P+ which is loaded to no greater, and in all likelihood substantially lower pressure is going to be problematic.