The only difference between the 7447 and 7457 is support for L3 cache. Given that none of the Macs shipping with these processors actually have L3 cache, that difference is irrelevant.
In the API to determine the CPU type, there is only an enumerant for the 7447. However, Apple's own CHUD tool identifies my CPU as a 7457, so maybe even Apple doesn't cleanly distinguish between the two.