My opinion is you are looking for lack of distortion and visible stress or lack thereof of at the rivets.

most of these when the cantle plate date code can be found are going to be 40-50 years old.

i have had used Brooks failures, on the old sprung roadster saddles. Both failed at the nose. One came with the bike, the other was $2. At a garage sale. All told, 6 years of service.

leather dressings at this point will be marginal to help comfort or extend service life.