Rong, the "serial number" system pre-dates any dealings I have had with B&S engines. It was a system that seems to have existed after the letter types but before the "code" was introduced, as far as I can understand data in the "Briggs & Stratton Repairman's Handbook for Older Engines 1919-1981". However when looking at the records for the serial number, I get the seemingly impossible answer that your engine was made in 1935:
http://www.myoldengine.com/engine-serials-search
