The Parker "51" Cartridge/Convertible Fountain Pen   Parker "51" with "51 Convertible" chalk marks  Parker "51" with "Cartridge 51" chalk marks    In 1961, Parker tried to capitalize on the success of their new convertible Parker 45 introduced the previous year.  The resulting model was essentially a "51" in most respects, with the exception that instead of a built-in filler, it now had the capability of using either a converter or cartridges and for the first time its famed finned collector was changed, as illustrated in the picture to the left.    The collector was no longer finned and seemed not to serve any purpose other than holding the redesigned feed.   The nib remained the same as in the other "51" versions.       The pen had a unique two-sided connector ring and a nipple coming from the feed, sometimes confused by the inexperienced collector for a broken aerometric filler.  The barrel is also unique, in that it has no breather hole either at the end of the barrel or on the side and is cut shallower and narrower inside and will not fit a regular full-sized aerometric pen.  Although advertised as more convenient, the success of this pen was limited, and it was discontinued late in 1963.  The reason for the limited success has been speculated to be that the cartridge gave the "51" an image of being disposable, something that had not been part of the "51" positioning over the previous 20+ years of production or maybe the redesigned collector affected performance more than expected. Examples were advertised in black, gray, teal blue, burgandy and navy blue.  The repair manuals of the time do not list a forest green version. There is also rumors of a Convertible/Cartridge "51" demonstrator.  I have not been able to locate or inspect a correct one yet.   The ones that I have seen so far have the regular aerometric demonstrator barrels, an easy pen to put together if you have the right parts.  If anyone has a cartridge/convertible demonstrator with no breather hole in the barrel, I would be very interested in inspecting/photographing it.