This is the Heavy Cavalryman - 5th Century AD that first appears in the Miniature Figurines 72/73 catalogue as PBC 142. I have added 'Roman' to the title - Minifigs taking it as read.
As with all the 'PB' range it is derived from Phil Barker's pair of books Armies of the Macedonian and Punic Wars and Armies and Enemies of Imperial Rome. PBC standing for Phil Barker Cavalry. This figure corresponds with illustration 42 in AEIR.
He is riding a horse coded HCH 2 - which I assume stands for Heavy Cavalry Horse 2. This looks like a later redesign of the contemporary |HCH 2. It's somewhat between the PBH and S series horses and the later H and A models and quite a handsome beast really. The code is stamped on the base in the usual way.
The model can also be seen on the 72 catalogue page for The Armies and Enemies of Imperial Rome where it is mounted on what looks like an S range horse.