yep, reinforcement brace under the caliper
it’s through both sides- note the light shining through from the outside hole
paint is rubbed off because I noticed the rusty bit and started sanding off the paint to see how bad the rot was, intending to touch it up to prevent any worsening. sanding just knocked a hole straight in the rotten steel
looks like the rust started from the inside, I fished out a bunch of rusty crud with a magnet
Edit: here it is pulled back a bit. The hole was drilled a few years ago when I realized it was a little rusty in there, clanking around- fished out a bunch of crud at that point too, then taped over it to prevent water getting in there and just sitting
