As far as I know these body threshold trim plates were never factory issue but could have been common dealer installed accessory parts. Since the body sill (outer rocker panel in particular) is tapered front to back, the one pictured is a right side part, wider on the right (front) end with a small downward flange on the outer edge. These could have been available from several aftermarket parts suppliers such as Amco and MG Mitten, as well as over the parts counter at your local MG dealer. They are still available today as new aftermarket parts.

Threshold plates are commonly made of brushed or polished aluminum, but might also have been chrome plated steel (on rare occasion). They usually have ridges that serve well to camouflage scuffs and scratch marks. Some may have a large MG emblem embossed in the center facing outward. They are most often held in place by two grommet head trim screws (see holes in picture). Once installed you probably don't want to remove them (and leave the screw holes).

Do check first to assure that there is space enough to fit these parts below the door shells. Some replacement rocker panels (over the past 40 years) have been too tall, so the door shell may be sitting very close to the body sill. Too close and you may end up with the screw heads scratching paint on bottom of the door shell (or the sill cover plate jamming the door).

The preceding page shows interior frame rail covers that are often used in conjunction with the threshold plates.