It's the retainer from one of the original Tele cup jack mounts. It's a piece of steel slightly larger than the hole so when it gets pushed in, it bends and puts pressure on the walls of the hole to keep itself in place. Then, the jack/cup mounts to it and theoretically keeps the whole assembly in place. Didn't work very well and required a special tool to work with, so they're often replaced with... just about anything else. In this case, it looks like a square jack plate was installed in place of the original cup assembly.
Easiest/safest way to remove it is usually to push it the rest of the way through. Trying to pull it out could easily damage the body with wood tear-out.