These are matrices—the pieces of copper that are the molds used to produce pieces of type—used for the typeface Coronelle Romaine. If you zoom in, you can see the nicks at the bottom of the matrices used to orient them in the mold and the individual letter forms, as well as ligatures (joined letters), numbers, and assorted other characters. Stamps made to illustrate this typeface can be seen here.
Matrices, Coronelle Romaine typeface, designed by Hendrik van den Keere, ca. 1570-1573.
Museum Plantin-Moretus, MPM.MA.062 (public domain)
