Basically, Triumph released the TR5 as an up-engined TR4A meant to fill a niche in the market for a model year or two until the new TR6 was ready to go. It's essentially a TR4A with a TR6 motor.
But 1968 saw the dawn of ever-tightening emissions regulations in America and the new fuel-injected, 9.5:1 compression, 2.5L straight six would be tough to bring into compliance. (To say nothing of the cost bump, estimated at as much as $600, on a car that cost under $3,500.) So the US market got the TR250, with a pair of Stromberg carbs and an 8.5:1 compression ratio instead.
According to the Wikipedia article, out of just less than 8,500 TR250s sold, around six hundred of them are still out there, and here's one.
.
