Gold Filled Caps with plain arrow gold filled clips, with pattern of closely spaced lines all around.  Note the "Parker" on the bottom of the cap, over the line pattern.  This was a unique sized cap introduced for the Demi "51" in 1947-1948 with no Blue Diamond guarantee.  One of only two patterns available. 

Canadian-made Sterling Silver caps with chrome plated clips, lined pattern with chevron band.  This combination of Sterling Silver with the plain arrow clip was only available in canadian production, around 1947-1948 for the vacumatic pens.  These caps all had a rhodium plating to prevent tarnish.  The finish did not stand up to wear too well, often wearing inconsistantly, and can sometimes be found with an uneven, unattractive appearance.  In addition, the soft metal is very prone to dents and dings, and finding caps in perfect condition is very hard.  

Rolled Gold Cap with gold filled clip.  English made, ca. 1949.  Pattern is groups of 4 lines and a wide space, all around the cap, with a wide cap band with "Parker" in the front and "51" and gold content on the back.  Note the unique shape of the clip, common on english "51"s of the era.

Less Common Cap