This interesting object represents the intermediate phase between the plaster sketch and the marble creation of a sculpture. The portrait was covered with dots so it can be translated into marble. Two pieces of marble arefixed on a surface to indicate the material of the final piece.
This marble bust was created by Houdon in 1780 and presented at the Salon. For comparison, see the plaster one owned by the Metropolitan Museum of Art, also with dots for the transposition into marble and of dubious attribution.