You are correct, the foot figure is the PB 50.
If you have an early copy of "Armies of the Macedonian and Punic Wars" by Phil Barker, the numbers etched on to the figure bases correspond to the drawings in the book. Minifigs used the drawings to sculpt the figures and assigned catalog numbers as "PB" or "PBC" plus the drawing's number.
The cavalry figure is figure number 9 on page 36 of "The Armies and Enemies of Imperial Rome" by Phil Barker (1972 version). His title is "Auxiliary Light Cavalryman of the Early 1st Century A.D."
In Minifigs catalogs from the 1970's, he is listed as...
PBC 109 Auxiliary Light Cavalryman of the Early 1st Century A.D.
Minifigs used the figure numbers from the "Imperial Rome" book but added 100 to list them in their catalog. So, figure number 9 is listed as PBC 109.
In later catalogs, the figures from the "Imperial Rome" book where given the "IR" and "IRC" designations replacing the "PB" and "PBC" designations and the numbers were changed. My guess is that the figure is now listed in the catalog as "IRC 26 Auxiliary Cavalry Early/Mid 1st Century A.D." .