There is more to it than just the brand/type of underpants he wore! There is also the size (under-sized will sound radically different from over-sized), the year of manufacture (NOS elastic or synthetic rubber?), how they were laundered, and the inherent interactions with other garments being worn.
Additionally, you seem to imply that there was only ONE pair of underpants employed during tracking of that classic song. I suspect there were multiple tracking sessions, followed by multiple mixing sessions; and I further suspect there were multiple underpants influencing the evolution of that great song over all those sessions. Is it better to track with loose underwear and then tighten them up during the mix, or vice-versa?
Not to mention the need of similar underpants information for the other members of Who, since they operated as an ensemble!
PS - on the other hand … how do you know he (they) wore underpants at all? Is there something in the production that leads you to believe he (they) didn’t go old-school au naturel?