300-WM, my friend, things are not always as they seem. Is there the possibility that a previous owner may have "worked on" the bars? Seeing hammers with reduced sear notches is not uncommon -- some shooters try their hands at "trigger" jobs. Perhaps someone tried an "action" job on your 870. You know, remove a little metal here and remove a little there, it's got to speed things up.
A lot can happen in 49-years. Of course, to those of us who are older than BigJim, a gun made in '63 is nearly new.