The Proctor "D" section is somewhat "pear" shaped in cross section, with the sides at the back fairly straight, unlike most of the other sections, which are more rounded at the back.
Proctor Masts web site
Proctor mast sections
DDD D D D D D D <=== Sides at back of section are almost straight on "D". D D The "E" is very similar, though slightly smaller in cross section. BBB B B B B B B B B <=== Beta, Epsilon, Stratos and other Proctor sections are BBB more rounded.Older Proctor masts and boom are typically gold anodized, while a light grey silvery color was normal after the early 1970's.
The "D", "E", Beta, and Beta Minus were common until the mid '70s. More recently, the epsilon in the early '80s, and the Stratos in the very late '80s and early '90s, have been common alternatives to the "D".
The Z-spar, a round section tapering at the top with a sail track riveted on, was normally anodized a silvery color. The similar Australian Goldspar, has a distinct two sections frequently of different colors. The Elvstrom masts were usually anodized black, and some models are flatter at the top, to induce sideways bend at the top.