Watch List notes:

UNCERTAIN. AR Denarius (19mm, 3.52 g). IV(?) countermark or banker's mark on a denarius of L. Farsuleius Mensor (Crawford 392/1a). Host Fine, c/m VF, toned.

From the Richard Baker Collection.

Possibly a private banker's mark, rather than the more civic or public countermarks. If so, it would be unusual for a banker's mark to appear in a rectangular incuse, rather than simply letters punched into a coin.