M4 is the date code for the building of the carb. M= the month, 4=day of the month. Your 3853s shows what year and engine this fits,1965 273. That carb was built on M4. Look at a calendar and each month is assigned a letter beginning in January. Jan=A, Feb.B, Mar.=C, April=D etc. So "M4"= Jan.4th, 1966. This is when your carb was actually built. This could be the original carb for your car if the build date of the car was after 1/4/1966. See how easy that was?