On Parker barrels, the space between the barrels and the ribs at
the muzzle end have visible triangular metal filler
strips, called keels. Keels were
tapered triangular wedges over one inch in length.
served several purposes. The keels soldered in place added
surface area to the barrels when they were soldered together
before the rib's were installed. The keel are on the muzzle end
because it was tapered and could be inserted and tapped in, or
pulled out, to help regulate point on aim. It would then be
trimmed off when the muzzles were finished.