From the 2019 Scott US Specialized catalogue, page 280:
"No. 4001b is a convertible booklet that was sold flat. It has a self-adhesive panel that covers the rouletting on the inside of the booklet cover. Numbers 4001c and 4001d are component panes of a vending machine booklet, which was sold pre-folded and sealed, and which does not have the self-adhesive panel covering the rouletting on the inside of the booklet cover."
4001a is a single stamp from any one of the other booklets 4001b is a booklet pane of 10 #4001a stamps 4001c is a booklet pane of 4 #4001a stamps 4001d is a booklet pane of 6 #4001a stamps