When it became common for men to go without a waist covering. My shirt takes 5 studs, and the bib goes all the way down into the waistband. It's evolved over time. Anyway, you should still only show three studs - I have a vintage set, with only three studs, and wear those as the top three, that show above my waistcoat. For the last two I use two of the cheapo studs that came with the shirt.
What kind of shirt(s) are you planning to get?
Does this also have to do with men's trouser waists getting lower as the years went on? Even with a waist covering (if it is a cummerbund, I guess) you'll end up showing more shirt than in years bast since even "regular" rise trousers are lower than in the first half of last century.
I also wonder whether this happened by accident: stud makers began including an extra stud (when they stopped being made from precious metals, most likely) as a spare and shirt makers interpreted this as a need to include a fourth hole.
As an aside, why not have buttons sewn on for the final two to reduce the risk of panic if one of the cheap studs gets lost?