I may have been incorrect on the usage (years) of the TVS, but the info came from Pontiac Enthusiast Magazine, and I just found it here on FGF.

The TVS-GM#3016754 is used on all 1968 V8's and on 1969 V8's with automatic transmissions. Also, several early production 1969 Ram Air III cars with manual transmissions were built using the TVS and the vacuum advance valve. The 1971 455 HO engine also used the TVS with either transmission.

In all applications, the TVS serves as a safety device to help prevent overheating. The switch is located at the front of the intake manifold, threaded into the coolant passage. There positions inside the TVS related to coolant temperature. Standard vacuum flow is ported vacuum from the carburetor to the distributor vacuum advance. Whenever the engine coolant temperature rises above 230 degrees F, the TVS switches the distributor advance from ported to full manifold vacuum. This advances the timing about 20 degrees at idle, allowing the engine to cool down to normal operating temperature. After the engine cools, the system returns to ported vacuum.

Either way, it's not necessary, but the restorers were paying a hefty premium for the 5-port job a few years ago when a friend told me to sell the two I had! I thought they were worth about $10, but he was right, they both sold for $125+.