I would guess they used braizer head rivets to begin with to reduce drag on the bottoms and just used them everywhere (flush head riveting is slow and difficult which means expensive, they only did that on the racing hulls). The original 5/32” rivets with braizer heads can be replaced when needed with 3/16” modified braizer head rivets and the heads will all match. Later Vagabonds (57 up) had round head rivets, I don’t know why they changed.
For a given shaft size, the Modified Brazier is the smaller one and it's virtually the same size as the next smaller shaft's Brazier head. e.g. The head of a 5/32" Brazier is the same as a 3/16" Modified Brazier and they use the same rivet set.
No act of kindness, no matter how small, is ever wasted. -Aesop