New & old 1991 Topps variations scarcity including backs (personal experience).

[steve B]

Don't forget the other glow backs, which are a very dark red under UV.
From the smallish sample size I have, they're not at all common.

There's a few other UV oddities, but so few I can't rule out something post production.

Cardstock that either reacts or reflects UV, showing sort of blue white ish, or with white fibers. Only a couple of each, and they could be fiber transfer from a reactive paper.

A glosscoat that reacts green under UV. Only one so far, but I haven't spent hours doing the fronts with the blacklight.