I will look through my brush axes and maybe it will help shed some light on that time frame?Plumb is definitely the first company doing whole-steel construction I've seen incidentally when scouring period documents for scythe-related info. In all the old publications I've seen Plumb in I see them using whole steel quite a ways back.
I have FRP as well as Y&P stamps.
According to Yesteryear...
FRP 1887-1893
Y&P 1869-1887 +-