1961 Chevy Impala heads

The casting number will vary with the parts suppliers and lot number.

As long as they are heads designed for your specific motor, they'll work just fine. But if you wanted to keep your original casting numbers (A Numbers-Matching Classic car), then you should contact the people who did your head work and tell them you want your original heads back.

Because what most rebuilders do is give you heads that where already rebuilt, and take yours as a core, for future rebuilding.