Your informant is partially correct. Judging from factory PR photos of TCs, TDs, and TFs, none left Abingdon with any mirror other than the center-mounted one on the scuttle. Nor are they listed as factory-installed items, according to the NEMGT's reference manual, "The "T"-Series MG Handbook".
Fender mirrors were a dealer-installed item (along with a lot of other stuff, like heater boxes, etc). They were mounted where that 19-year old mechanic apprentice felt like drilling a hole in the fender. No standard location.
My fellow "T"-series drivers who have fender-mounted mirrors pretty much agree they're unusable, due to fender vibration and being constantly knocked out of position by people walking past the car. And, it's a two person operation to re-align them. They do look good, however ...
Another option (which I use) are mirrors mounted on the windshield posts. Vibration free, and easy to adjust from inside the car. Moss sells the official set ($), and a private party sells a very nice period-looking set. Again, not factory original, but it looks good on the car!