It's a 1974 Pearson 26
I think there were 3 masts used on the Pearsons that year.
This
rig has the cutout(about 2"), and a "door" which pivots on a pull-pin.
A second pull-pin is inserted to "lock", the door in place, thus disallowing the slugs to fall out.
I can buy a conversion or juryrig a replacement . . . I'd rather have an original/matching part.