According to Muckley, the shorter version, Centaur G, was to be used to launch DOD payloads, the Magellan spacecraft to Venus, and TDRSS [tracking and data relay satellite system] missions
...what the hell was the idea there?
I watched this guy go up from Vandenberg, from the same pad (SLC-6) they built for those DOD shuttle missions prior to the Challenger disaster. It turned out to be a KH-11 Block IV, the one we used to confirm bin Laden was in Abbotabad.
The problem there is that if you need a big booster, it's probably for a big payload... because so far as can be reasonably surmised, the NRO has no need to conduct surveillance on alien worlds (yet). Which means there isn't room for both the satellite and the booster, particularly if you're having to do jackass stuff like ditching the galley to make room for rocket fuel.
This seems for all the world like yet another attempt to justify the shuttle and prevent the development of decent launch vehicles like the Delta IV Heavy. Which - apples to apples - costs 1/4 as much as a shuttle launch.