The Stinson may not have been originally equipped with.
Should be installable afterwards, not?
The Stinsons I am aware of, like the Cessnas and some PIpers of that older vintage used a metal on fabric cam type seatbelt. The stiffer than we see now seatbelt webbing passed through the serrated cam type buckle and was pinched there by the force to remove it.
THe problems were found to be two: In a severe crash with big loads, it could get pinched so tight by resrating forces that the occupant no longer had the strenght to open the buckle. Secondly, if the cam pinched webbing got wet it would swell up and jam worse, again, the occupant could not open it.
In Canada, initiallty for flaotplanes, then soon for all planes, these fabric on metel seatbelts were prohibited. in favour of metel to metal.
Shoulder harnesses can be installed under AC43.13-2B as "specified data", and are vitally wise....