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Single Jewel Vacumatic - These seem to be the most common variety of 51's. Pens from 1942-1945 are harder to find due to war time restrictions on production. The 1946-1948 pens are fairly common though. They come in black, cedar blue, dove gray, and cordovan brown. There is a demi model that is shorter than the standard pen. The demi's come in black, cedar blue, and cordovan brown.  The 1948 pens have both the arrow pocket clip and the blue diamond pocket clip. Earlier models originally came with the blue diamond pocket clip.