See Museum Registration Methods, Chapter B, A Classification System for Art
Objects.  This describes the system in use at the Walters Art Gallery.
Their accession numbers begin with a two digit classification number
followed by a running number; for example 37.91 is a painting (
classification 37) and was the 91st painting to enter the collection.  The
system worked very well when I was on staff there.  It was handy to be able
to tell at a glance how many of a certain type of works the museum owned.

Beverly Balger
Registrar
Palmer Museum of Art
The Pennsylvania State University