I've only ever heard Tab from students at Oxford referring to rivals (mostly sporting) from Cambridge. Not sure its common in other contexts though.
I do occasionally hear it mispronounced as "Cantabridgarian".
Also in the 80s there was a small computer company based in Cambridge, England called Jupiter Cantab. They made the Jupiter Ace computer, a semi-clone of the ZX81 with Forth in ROM instead of BASIC.