...wonder if it was a nail cup !! I mean they didn`t have nail aprons back then !! Needed someplace to put nails while hammering !!...
Getting sidetracked here, but...
I'm curious as to why you feel they didn't use nail aprons at the time? Even the Book of Hours from 500 years ago depicts workers and tradesmen using aprons of some sort, including examples with pockets across the front.
My thinking is...anyone smart enough to build a house probably figured out where to carry his nails
.