The engine code is stamped on a smooth flat boss on the block just ahead of the distributor. The first digit indicates the year, 1978 would be represented by a 2. the next two digits are the month, then the engine type, a 258 from that time period would be a C, a 232 would be a E or with certain engines a F. the last two digits are the day the engine was built.
IE: 210C12 would be a 258 built on Oct.12th 1978.
After 1980 the first digit matches the last digit of the year so 210C12 could also be on a 1982 engine.

AMC engine date codes are explained HERE . <img src="/forums/images/graemlins/cheers.gif" alt="" />