Be careful, my information indicates that in 75-76 the TR6 alternator has a 17mm shaft diameter, and it takes a 6203 bearing.
Edit: Disregard the above info...
Sorry, but I double checked, and and the shaft size on the 75-76 TR6 alternator is 15mm. The 6203 bearing that it uses instead of the earlier 6202 is just larger in the OD, not ID (shaft size). For some reason I got dyslexic and thought 6302 bearing instead of 6203 that I was seeing when looking up this info.