The standard (non-Coke Bottle shaped) target stocks with the diamond insert around the the grip screw escutcheon were sold by Smith & Wesson until some time in 1969. Although the M28-2 was shipped with magna service grips as standard; back then target grips, hammers, triggers and sights could be special ordered. Further some dealers would swap grips or other "target" options after deliver from Smith & Wesson or the wholesaler based on what was currently in demand in their area. I do not believe that target stocks were ever numbered to guns at the factory. The diamond target stocks on your gun are period correct for it's manufacture date but the only way to know if the gun had then when shipped from the factory is to "letter" the gun.
Bruce