What is this?

Looks like the OEM rotor. The OEM caliper would have been mechanical, cable operated. Someone has converted it to hydraulic. It's probably a pocket bike or pit bike caliper and master cylinder. The caliper isn't ideal for its location/position because the bleeder should be at the very top; it isn't too far off, though, and the brake could probably be bled; I'd lift the front of the bike, getting the bleeder higher, to bleed it.