CT stands for Cut Throat, the base coverstock name for the early Vibe series. It came in pearl and solid versions, like most coverstocks.

CT Plus was the nomenclature for the coverstock of the Onyx Vibe:
http://www.hammerbowling.com/products/retired-balls/onyx-vibe
It was believed at that time that the Plus stood for a tiny bit of particles added to the base CT resin formula. The Onyx was slightly stronger (read earlier and more hook, but just slightly) than the original solid, the Blue.

All this is just my memory of what went on 10+ years ago, without stone cold facts to back it up. (I don't recall whether Hammer ever publicly verified this.)