What The Heck Is It?

This image was passes out yesterday in the Philatelic Librarians group I belong to, so I thought I would pass it on here with the question…what the heck is it and how was it used?

The question was answered in the Librarians group but I'll hold off with posting it. First person with the correct answer (and who is not in the Librarians group) will win a set (3 Pairs) of these Staehle HPO Covers
Prize for first correct answer

Don

All I can think of is that this is a "holder" for a canceller "month" "date" and "time" slugs that are used to cancel the stamps. That is what I think the holes would be for - to hold the slugs for each. Other than that, I have no clue!

Close enough, Dave wins.
It is an International Postal Supply "Flier" canceling machine type holder. The slots hold the pieces of steel type for hours, dates, months, and years. The center hold is for a screw which secures the type holder within a slide-out tray under the canceling machine. This round type holder is secured in the swing-out tray shown in the image below.
Many thanks to Frank R. Scheer, Curator Railway Mail Service Library

Don

Here is a similar one for a different make/model machine with different bases to each slug.

That's really cool!