I found this
n 1968 Dick Higgs began work on the ‘S’ range (S stood for “special”) of figures, now considered by many to be the first “true” 25mm figures though they are actually nearer the 24mm mark. Thicker set than their predecessors, but by no means as chunky as the later 25mm figures, the ECW range is shown off to good effect in the photo section of Charles Wesencraft’s “With Pike And Musket”. The first figures, ancient and medieval subjects, appeared in May 1968. At first cavalry were one-piece castings but this was changed in 1971 when a new “dismountable” series was introduced.
The ‘S’ range figures too were originally sold and advertised as 20mm and it wasn’t until a year later that they came to be officially tagged 25mm. The S range figures were priced at three old pence more than their smaller foot-slogging brethren, one shilling more in the case of cavalry. The figure code, traditionally scratched on the base of the figure, is the same as that of the earlier models with an “s” added after the number (for example, FN47S). When the HO/OO ranges were discontinued this suffix was dropped.