That looks like a hole that has worn out into a slot?
Happens with aluminum
masts, metal fittings or even synthetic lines can cut the aluminum
. I would look for a couple of piece of stainless plate, maybe 1/8" or 3/16" thick, have them bent to match the curve of the mast, one on each side. Then have them drilled out, one big hole in the center to "replace" the slot in the mast, and some smaller holes around the edges so you can have them pop-riveted to the mast. Appropriate rivets to prevent galvanic issues with the two metals, and between the plates and the mast.
Or, of course, you could tap the mast and screw the plate down, if you or the machinist got together with the mast.
No biggie. Just lap a plate over each side, don't worry about welding on the mast.
There are actually stainless plates sold
for small masts, to reinforce in-mast halyard exits that have been chewed out the same way. Slap 'em on, pop rivet, problem gone.