As poolboy's link indicated, those were part of a recall campaign where the rear tail lamp lens reflector did not meet federal standards and those reflectors were fitted to supplement the existing tail lamp reflectors in order to bring the cars up to the federal standard. It was on campaign CCL693-TR6 and as far as I know that recall campaign bulletin is the only place where the Triumph part numbers for the plinth (155750) and reflectors (629623) plus the screws ever showed up. The plinths are long NLS but the reflectors are the same as those used on the rear wings of the 1969 MGB, MGC and Midget under Lucas part number 57098.
Early TR6 models were also caught up in a headlamp height recall that resulted in new front springs being fitted if the headlamps centers were too low. MG was caught out on that as well with a similar recall issued for the MGB and B/GT to replace front springs if required.