It is the manual embossing by cold hammering that was adopted in the workshop. 

Hammering of various stamps and chisels, on vegetable tanned leather exclusively, which allows the contours of a design to be lowered and textured, by creating a play of reliefs which is similar to an augmented 2D ... 

A sort of inverted version of engraving, that digs the motif into the material ... and a slow, precise and technical practice which does not admit of error!