This is Bio One, a 12-page digest-sized booklet published by TSR in 1974:
It’s a rules supplement that “provides a complete and accurate system of damage location and effects for any gunpowder period of warfare.” In other words, it tells you where the bullet hit, how badly it hit you, how much bleeding you’re currently experiencing, whether anyone can stop that bleeding, and so on.
It includes this amazing bit of business: the Abdominal Wounds Chart, which divvies up a torso into numbered sections, and each section having a subseries of multiple possibilities of injury (or lack thereof):
For example, if you’re shot in section #7, a randomized die roll could specify your injury as lung damage, or as rib damage, or as both, or perhaps, if you’re lucky, as no damage at all (like it bounced off your “medicinal” flask in your pocket, or your Bible inexplicably bound with solid steel covers).
It’s an interesting, if almost too-exacting approach to role-playing gunfire injury…it wouldn’t surprise me if similar systems weren’t implemented in later games. And if they were, I’m sure someone out there will let me know in the comments.