I looked up on Richard Binder's website the dating system for Parker, but my 51 does not match any of them. I live in Brazil, so I guessed there was a possibility that the one I have was made in Argentina with the machinery Parker abandoned, as stated on Mr. Binder's website. But the cap clearly says that it was made in USA.

Well, my pen has all of the characteristics of a Mark I pen: rounded barrel end, metal filler with a plastic finish and a broad double clutch ring. The clip is the short arrow with no jewel or incsribing of any kind. But there is nothing on the barrel to identify its date. The only clue I have is an engraving on the cap: I/IO, or 1/10, I'm not sure how to read it. What does that mean?