Around 1972 Triumph went to a PI head for all TR6 models, and made a lot of little changes to the engine to meet stricter USA emissions requirements. It has the more wildely spaced intake ports and better flowing intake passages, as well as having valves of a slightly larger size. These changes, coupled with a revised cam profile and CD2-SE carbs (the ``tamper-proof'' ones with adjustable needle height) improved emissions and actually increased the horsepower rating a bit, from 104 at 4500 to 106 at 4900 rpm.
Of course those not fooled by horsepower ratings will notice that the figures point to a lower torque across the usable rev range, but it is a bit flatter in the midsection. Not a bad job, really, on the part of Triumph.