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)
".. 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)
1984 Fast/Nicholson 345 "FastForward"