One other possibility that only recently crossed my mind (and not based on any particular knowledge of the day) is this:
Up to about 1971 power figures quoted by manufacturers were based on the SAE (imperial) bhp norm.
From 1972-on this was replaced by the kilowatt as the official power-measuring unit. Most manufacturers retained bhp for commercial and advertising purposes, as many customers were not familiar with the use of kilowatts for engines. But they did switch to using the metric (DIN) bhp instead of the imperial SAE norm.
The difference, if looking to other Triumph models could be up to about 5-6% less power output with the new DIN norm. See for example:
- Triumph 2000 saloon: 90 SAE bhp vs. 86 DIN bhp
- Triumph 1300 TC: 71 SAE bhp vs. 67 DIN bhp
- Triumph GT6: 104 SAE bhp vs. 99 DIN bhp
- Triumph TR6 carb. (US): 106 SAE bhp vs. 101 DIN bhp
As the Sprint engine was designed in this period, could it be that at first an SAE bhp figure of around 135 was attained, but at the time of introduction in ’73, the DIN bhp was the norm.
By interpolation, the original 135 SAE bhp of the Sprint could be calculated back to 127-128 DIN bhp. So same engine, but different bhp calculation and the name “135” could no longer be kept?
Just a thought