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Double Jewel - The first Parker 51 pens had jewels on both ends. Beginning in 1942 the bottom jewel became an extra cost option until 1947 when they were dropped. The initial colors were India Black, Cordovan Brown, Blue Cedar, and Dove Gray. Nassau Green, Yellowstone (Mustard), and Buckskin Beige were added later and are a great deal harder to find. The Nassau Green and Yellowstone are only found in the double jewel variety. Normally all double jewel 51's have a blue diamond pocket clip. All colors have a matching pencil but they do not have the blue diamond clip.

The double jewel 51's also include the first year models. These pens have four main things that help identify them. Most have aluminum jewels but some have only one or in some cases none. The imprint and date stamp are at the top of the blind cap for most of the year. The pens from the last few months have the imprint at the clutch ring. The earliest pens have the aluminum speedline filler and the later ones have a plastic filler. First year pens can have any or all of these items but they are more collectible if they have all of them. One last item is not seen unless the nib is removed. The first nibs do not have any markings on them. Later nibs are marker Parker and most are dated.