Both PB ranges were based on Phil Barker's books Armies of the Macedonian and Punic Wars and Armies and Enemies of Imperial Rome. This figure is taken from illustration 28 in AMPW.
The 's' designation was used to distinguish the newer 25mm style of models from the preceding, smaller 1/72nd range. The transition was soon complete and the 's' - special - was dropped from the figure codes thereafter.
The base is stamped PB 28 on the upper surface in the usual manner.
This example has an especially thin spear, and I wonder that they managed to cast it! Many will be broken. The same model appears with the usual - and I assume original - much thicker and robust spear in the style of the preceding 'S' range Ancients.