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.  

Keels 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.


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