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Parker 51- Demonstrators come in a variety of different types. The top pen is a first year model with aluminum jewels and the imprint on the blind cap. These are factory engraved "Demonstrator" on the side of the barrel. The hood is clear and the collector is made of a transparent red plastic.

The second is from 1947 and is fully transparent. These have a standard clear collector. Similar pens as well as pens  in transparent colored plastics are made in Argentina. To be considered a USA pen it must be correctly marked with Parker USA impressed by the clutch ring.

There are two additional versions that I am aware of. One type has a clear hood to show the collector. These are found on Vacumatic fillers as well as the Aeromatic fillers. These pens are usually not marked in any other way. The last type is a clear Aerometric filler.