Prior to 1919 (or serial number 316,648) .38 Military & Police model cylinders were not heat treated. On March 7, 1942 they discontinued the practice of heat treating/hardening the end of the yoke barrel on which the cylinder revolves. During the late 1960's and 70's the demand for revolvers resulted in some slippage in quality control, in particular what would have been considered excessive cylinder end shake in earlier times.
Cylinder end-shake is a condition that once stated, tends to increase rapidly, especially if high-end loads are used on a regular basis.
Plus-P loads were created for law enforcement agencies that wanted more power in their .38 Special revolvers, but could not go to .357 Magnums – usually because of politically inspired restrictions. It was generally understood that extensive use of some Plus-P loads could cause accelerated wear on some .38 revolvers, but the agencies involved didn't care, as they simply planed to turn in their old guns and buy new ones if the ammunition they were using took an excessive toll.
In steel, K-frame revolvers the steady use of certain brands of Plus-P ammunition may accelerate cylinder end shake, especially if some back and forth cylinder movement was present when the gun left the factory. In pre-1919 revolvers there is a remote chance that one might expand the chamber(s). All K-frame revolvers in sound mechanical condition should be safe to fire with .38 Special ammunition loaded to 16,000 PSI or lower, and in particular the so-called “cowboy” ammunition loaded with 158 grain lead bullets loaded to a muzzle velocity between 700 to 800 FPS out of a 6” barrel should be fine. These of course are not barnburners in any sense of the word.
Pre-World War Two Military & Police revolvers are usually superb and highly accurate shooters that hit precisely to the point of aim when they are used with the ammunition they were intended to use. The Old Fuff is determined that the ones he has will be passed on to future generations in much the same or better condition then they were when he got them. If and when more cartridge power is required it is easy too obtain revolvers that are rated for Plus-P ammunition, or even better, obtain one of the approximate same size chambered in .357 Magnum. Others can make their own choices.