Previous dating for your M&P is correct, it likely shipped late 1919-early 1920. For any of the earlier M&Ps, lead bullet standard velocity loadings will always be best. That's the only ammunition that was available at the time, and there is really no good reason to use anything else. If the grips are not numbered to match the SN of the gun, they are not original. S&W was transitioning at the time from grips which had small recessed gold medallions to those with a rounded convex top without medallions, so I suppose either style could be period correct for yours.