I would say that is a built-up cornice moulding (i.e. built in situ from multiple mouldings) and if timber it would probably never have been sold as a single piece - if only because it would siimply be too heavy to loft it into position
Having reproduced and repaired a number if similar structures I'd say it comprises an ogee cornice moulding at the top which is fixed onto a hollow box structure.
Below that us a smaller rectangular piece, quite possibly just planed solid timber. To the underside if that a solid ogee moulding is fixed
The bit you are interested in is planted onto the front of that lower rectangle and is referred to as a dentil moulding (dentil= "teeth"). They can have a tendency to shed some of the small blocks over time. They are simply made up by (in this instance) by planing two different cross sections of timber, crosscutting to the desired lengths and glueing and pinning them in position.
So they normally can't be bought but they are simple to repair or extend