Gold Filled caps with gold filled clips.  Line pattern of nine lines, space, nine lines, repeating all around the caps, and plain cap bands.  Gold filled content at bottom of caps.     Common Caps

Gold Filled caps with gold filled clips.  Line pattern of group of five lines tapering up to one point near the clip, repeating all around the caps, and plain cap bands.  Gold filled content at bottom of caps.  These caps have deep engraved lines unlike the later aerometric series tapered line caps.     Common Caps

Gold Filled caps with 14k solid yellow gold clips in "window pane" pattern.  Model was sold as "Heritage" by Parker.  Line pattern of horizontal and vertical lines throughout the caps, and plain cap bands.  Gold filled content at bottom of caps.   Less Common Caps

Smooth 14k Solid Yellow Gold caps with 14k Yellow Gold clips.  No pattern on caps.  Marked 14K on caps and clips.  All 14K capped "51s" were marketed by Parker as the "Heirloom".   Less Common Caps   Interestingly enough, this newly discovered cap on the right, out of Germany, shows what would appear an imprint on the cap for export.  The cap is marked "585" instead of the usual U.S. imprint of 14k, probably due to European laws concerning gold content.