Hi Tom:
My comment is purely my own opinion. TR2 engine numbers ran quite consistently a bit higher than the TS number due to a certain number of engines going to other cars, like Dorettis or Morgans or whatever. For example, my car is less than 10 away from this one in TS number (mine is TS 1911 LO, this one is TS 1920 L), and my engine is TS 2067E. That pattern does not fit this car, as the seller claims the engine # of TS1721E is original to the car. OF COURSE nothing Standard Triumph did was perfectly sequential, and I agree it is possible the engine was original to the car. But consider this - a fairly inexpensive Heritage trace on the TS number would confirm if the engine is original. The car was owned by an avid TRA club member who would have known how to verify, and the car is obviously a great car with a high dollar value, so why not verify? Again, just my opinion.
Dan