Gijs is correct. If memory serves me correctly the T500 is 120mm in diameter and the GT500 is 108mm in diameter. Also the cases are adjusted accordingly to preserve the same crankcase volume. The last difference that I know of is the points (left) end of the crank is of a different length. It changes from country to country (I have found) but in Canada the 68 is a T500 and the '75/'76 a GT500.