I have recently received these specimens. These are some of the scarcest printings, and are part of my plan for a beginner's collection. I had been wondering how long it would take me to come across some of these stamps, and luckily it took only a month since I began to track this beginner sample.
The 20p1E1 (the very first printing, from 1936, and this one even has a 1937 postmark!)
The 20pCL (clay isssue from 1943, notice this specimen is postmarked in 1945)
The 5pNGR (unwatermarked grid, in my opinion somewhat over-priced in Scott)
The two high values from the 2C printing (straight rays, notice the very bright colors)