Casting date on an FE block is usually under the
oil filter pad visible from the bottom and it is cast into the block. The stamped # is assembly date of the engine and is usually on one of the machined bosses (probably the left front one below the exhaust port area). The date is year, month and day, like 6A17 (Jan.17,1966) same for the casting #'s so the 20F is probably another # (since there was no 20 year)