Well,, the places on the bottom look as if it had something else there and it broke,, but what I don`t know !! If it happened while house was being built,, wonder if it was a nail cup !! I mean they didn`t have nail aprons back then !! Needed someplace to put nails while hammering !! A small wooden piece on each side would have allowed it to slide or sit on the 2x4 if its wide enough !! Didn`t have to climb up and down ladder so often that way !!
Oh well,, that's my best guess,, that would also explain the leather !!