MOS-P is likely 1950, (and for Sears).
MOS-S would be 1953, because sometimes, "S" (e.g., in MOS) is for Sometimes-Sears, on Murray-built bikes.
Other times, the "S" stamp is a date code.
Murray may have re-used the "S" date code in 1960, but the difference between the 1950's and 1960's...