The T-Vac was factory installed for 57's with automatic transmissions. The Hydrovac was also a factory installed Regular Production Option for models equipped with standard transmissions (see 57 Shop Manual 5-29). Both T-Vac and Hydrovac were RPO 412. The 1957 Factory Assembly Instruction Manual shows the T-Vac RPO 412 Sheets 1-4, and the Hydrovac on Sheet 5 (and states that the Hydrovac is for all models except those vehcles with an Automatic Transmission). The Hydrovac, as stated earlier, was available as a dealer accessory for either the Standard or Automatic vehicles in 57.
Larry