Prescott was 64-bit from the first stepping (C0), but EM64T was disabled with fuses. It wasn't just introduced for E0, and it was disabled in the G steppings that were 32-bit the same way. That big a change is too large to do in a mask change (the letter incrementing in a stepping indicates a mask change, the number a metal change) rather than a microarchitectural one. There are probably engineering samples of C and D steppings floating around out there that don't have those fuses blown.

The same is true for the LGA775-based 32-bit Prescotts. 64-bit disabled by fuses.