... and at one time there was a recess to receive the litho (or photo) as seen on the two examples below.

The piece on the right is from Casino Municipal-Nice (early-1900s), and that's a picture of the joint in the incused space. The piece on the right is from an unknown location, and the reverse has a metal filigreed/die-cut-type inlay (probably 1920s-1930s France.

JB