dlear/476......They are stamped. If cast in they would have shadowed casting marks around the data from the "casting slugs" used to make them and allow them to be changed at will by the foundry. Casting slugs look very much like the lead ones used by printers when they "set type" but they are made of harder metal than lead that won't wear out.
Something I have always wished is that all replacement parts have the actual part number of that particular part stamped, engraved, or whatever method worked best permanently written on each part. That would certainly make things easier later..........Bruce