Progressive proofs 1939 $5 with full gum.
Proofs of the 1939 issue with um full gum. These are pre-prints which were used for colour and quality evaluation before full production. Other similar items for each issue of stamps could include essays, printers' samples including composite sheets, sample cards, presentation sheets, die proofs, progressive proofs, completed proofs (as above) etc etc. It gets very involved both in time and costs. Thankfully, not all issues would present the same challenges.