This is basically a myth. It apparently started as a result of calls to S&W Customer Service concerning use of +P ammunition in, specifically, .38 caliber revolvers. For some reason it became common practice for S&W to give the answer that +P was acceptable in any model-marked S&W revolver. Why this point was chosen is unknown, probably extreme caution. Because of the recommendation based on this date there have been those who have assumed there was some basis in fact, and it must have been because of some change making revolvers stronger at that time frame. 'taint so.
The truth is that when the 2" Military and Police revolver was originally introduced by S&W ca. 1936-7 it was advertised at that time as being acceptable for use of the .38-44 High Velocity cartridge originally introduced for the N-Frame Heavy Duty model. This cartridge has been reported as having developed ca. 25,000 PSI and higher, 25% more than modern .38 +P factory ammunition. At that time there had been no change in metallurgy for the M&P since 1920, when S&W began heat-treatment of cylinders for greater strength and resistance to battering of the bolt stop notches, mostly the latter. Up to that time cylinders were left in the annealed state and had proved fully adequate for standard pressure loads.
Over the years since hand loading with smokeless powders has been the norm there have been many "Recommended" loads published in various manuals and other sources which have developed pressures far in excess of modern .38 +P loadings. Millions, if not hundreds of millions, of these high pressure loads have been fired in S&W K-Frame revolvers, even those made prior to heat-treatment of the cylinders, with little obvious detrimental effect. Sometimes cylinders have been bulged, but rarely. Some guns have been blown up, but usually by inattentive reloaders who have thrown double or even triple charges into cases while loading. Catastrophic failures of revolvers are extremely rare with what have been considered "normal" loads, even those greatly exceeding currently published loading data by various bullet and powder manufacturers.