From my Samples, Singulars, and Somethings Completely Different collection -
This is a home-made leather arming jacket with an arm harness attachment strap running across the shoulders. I bought it back in The Dawn of Internet Time from someone on the Armor Archive. The mannequin torso wearing it has a 42" chest, and I approximate that the jacket will fit in the 40-46" range. I've never worn or used it, it's been on a hanger in my workshop for an Epoch or so. You're a lot more likely to use this than I ever will.