This is the Rhodian or Agrianian Slinger that first appeared in the Miniature Figurines 72/73 catalogue as PB 12s and subsequently in the 75 catalogue as PB 12.
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 12 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 12 on the upper surface in the usual manner.
I didn't have a raw metal version of this figure and someone has clearly made a start on this one by painting his sling. Why it all ended there we shall never know!