Exactly what we had on a former boat.. the top block for the vang had that 'nail head' fitting that slotted into that keyhole.. as long as the vang had some tension it stayed put.
Is this an originally roller-reefing boom set up?
".. 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"