One of our former boats had this same fitting - it was a keyhole slot for the vang, the tackle for which was connected via a "nail-head" type of stud that slotted into the fitting.
This was on a boat originally fitted with roller boom reefing... I believe it was to allow the vang to be used for a full main, but quickly removable to facilitate rolling the boom for a reef (but ironically leaving you 'vangless' when you perhaps needed it most)
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)