I have recently aquired a MK1 STAG which in nearly all aspects seems to be an original UK vehicle right down to the Warranty Engine replacement so common with the Triumph V8.
So is your car a Federal (LE) or UK (LD) model? A 1973 car would be considered a MK II model here, as they had al of the most important upgrades to cooling, etc.
But the Door Mirrors which should be Britsh Leyland Standard parts as used on all 1970's Triumphs and MGs, in this car were once Remote Controlled from a lever inside the car. The lever mechanism is still there on the Offside door, but the mirrors have been replaced with some which just don't work with the car.
I have been, possibly unreliably, informed that US spec cars had remote controlled mirrors.
Is this the case?
Since we drive on opposide sides of the car and road, and don't use the offside/nearside terminology, let's avoid confusion and refer to the left and right sides of the car. We Americans have the steering wheel on the left side, you English Chappies have it on the right. So are we talking about the left or right door mirror you're missing? You say it's a UK spec vehicle and it's the offside mirror. That's the left mirror, correct?
Back to the question. Some early Federal cars had fixed mirror very similar to what I see in Taylor's "Original" book on UK cars. I have a parts car (LE1594) with the early non remote mirror. Most later cars did have the remote adjustable mirrors. All I've seen have only a driver's (left side) mirror.
If so are there any suppliers that can provide me with a US Near Side external assembly?
Okay, now I know we are on the same page. The news is good and bad. I don't think there is a source for new mirrors. That's the bad news. The good news is that these mirrors were not unique to Triumph. They were used on other vehicles, most notably some vehicles from AMC (American Motors Corporation, long since subsumed by Chrysler). You could find these mirrors on some Matador, Ambassador, AMX and Pacer models. If you can find suppliers of used parts for classic AMC's (not the most popular of classic vehicles) you might find a mirror. EBay is another option. Search on "AMC mirror".
The other bit of good news is that driver's side (left) mirrors are much more common that passenger side (right) ones, so you have a chance.