Note that the 1957 "J" code refers to a 235 Thriftmaster with updraft carburetor. In 1958 and 59 (and maybe later?) the "J" code simply meant regular production Thriftmaster 235.
Kelvin,
As waldo53 suggests, post more casting codes if you want to try to figure out the year of the engine. As Randy suggests (and, if the valve cover has four screws/bolts holding it to the head), you have a non-original, later, high-pressure 235.
