I've never seen that tab, and I thought '80-86 thresholds were Aluminum anyway. I didn't think they became plastic until later. And I thought L&R were always identical.
That tab would interfere with the wiring that runs under the threshold, so it doesn't seem like that's the right part for these trucks.
My truck is a '81 F150 and it has the plastic ones.
1981 F 150 Custom 300 ci with a fully rebuilt 1968 240 head Carter YFA T-18 3.25 9" rear 2WD
dual gas tanks
1990 Lincoln Town Car 5.0 AOD
Home town Mc Kenzie, TN
The advert, which has ended, was for "E0TZ1013208C" and it was correct for our trucks. There were both aluminum, which fit both the right and left sides interchangeably, and the black plastic ones which were specific to the side. Both were used throughout the Bullnose era. All that's shown on the Interior/Sill Plates page.