It appears to be a stereotype mat, probably for a printing firm since it seems to be a listing of type specimens.

Edit: I just realized it is only a listing of faces but not in the type itself. Anyone who speaks Dutch might be able to figure out what kind of firm used it.

The Enschedé Font Foundry. More like the back cover of something maybe: you wouldn't put your phone number in the middle of a front cover, would you? Still, funny idea, printing the names of all those faces in Spectrum. (The bottom line means "Set in Jan van Krimpen's Spectrum".) Reminds me of the thread with the poster for the Helvetica film set in Trajan:)

I agree that the item on the left is a matt, and the casting made from it is on the right (unused, or cleaned exceptionally well).

Perhaps this was a printshop advertising it's catalog of available faces. The limited number (to our eyes) is probably about right when one had to buy lead type, or lino matricies.

In fact there are two parts: positive and negative.
On the left is a made of plastic and there were other examples, but in worse state.
On the right is made of lead. The was another piece there, but it was broken.

TEFF is one of Hollands "top" font foundries. Their quality has always been high and still is. The museum is more like a secondhand-shop... Strange...

Does anyone know the book? Could it have to do with 'embossing'?

I only found some names via Google, no images (bookcovers):
Spectrum Romulus Lutetia Romanee Open Kapitalen. Lettergieterij Joh. Enschede en Zonen. Haarlem, Holland. z.j. or.omsl. Rijk geillustreerd.

Here is another edition, from the 70s. So it seems this one should also be from a font specimen. BTW, I believe the historical Enschedé Foundry is now defunct. The name was resurrected by the new, up-upmarket, digital foundry (and the quality, but with new fonts).

Today I got a response from Museum Enschedé in Dutch though:

Geachte heer,
Het linker deel is een zogenoemde "afpersing" van los zetsel in een speciaal soort hard papier, een soort "Prespan" en dient om daaruit een cliché te gieten. Het cliché, rechts, wordt dan gebruikt voor de druk, in dit geval vermoedelijk voor een advertentie.
Hopend u hiermee iets verder geholpen te hebben,
Met vriendelijke groet,

[removed name]
Conservator Museum Enschedé