I have been doing a bit of research, as you do in lockdown, or in my case retirement as to which TR6 had side reinforcement fitted into the doors - see image attached. The most comprehensive detail that I can find is in one of the Moss Parts catalogues that gives commission ranges for the models with them fitted the Triumph parts book not providing any detail.  CR5001 onwards with cut out for door pull -  part numbers 63483 & 63484 CF1- CF12500 with no cut out for door pull - pa