The printer had produced the booklet on a clamshell platen press. I watched a YouTube video that showed such a press in action. The press operator first smeared printing ink on a round plate at the top of the press. Hot metal type (raised lettering rather than the flat typesetting plate of an offset press) was locked up in a “chase” and supported vertically within the press. Opposite the hot metal type, a flat surface held a sheet of hand-made custom printing paper.
When the clamshell press operated, the type section and the paper section of the press opened away from one another on a hinge. Ink rollers rolled up and across the round plate at the top of the press to collect ink and then rolled the ink down and across the custom printing plate containing raised type, line art, and illustrations. Then the press operator inserted a sheet of printing paper in the press, and the “clamshell” closed again. The intense pressure of the inked type block pressed against the paper (and its metal support backing) created the printing impression. Then the press opened, and the operator removed the printed sheet and replaced it with a new, blank sheet. Then the rollers inked the type for the next impression, and the process repeated itself.
Read more at https://www.printindustry.com/blog/2013/04/custom-printing-a-sample-of-the-art-of-letterpress/