You could probably study a degree course on Foden cab designations. They were never easy to understand, especially as they were technically not the model number, which was equally confusing.
However, hauliers and salesmen alike soon referred to vehicles by their S-prefixed designations. The first Foden cab designation was the W2 in 1932, the year it started to sell diesel lorries. But from 1935, the S2 designation was used. Then came the S9 a year later and the S10 in 1938.
Over the years they were followed by the S15, S18, S19, S20, S21, S24, S30, S34, S35, S36, S37, S30, S39, S40, S41, S50, S51, S55, S60, S70, S80, S81, S83, S85, S90, S93 and S95. And in among that lot was the FE4/8, which had no S designation nor did the early half-cab dump trucks.
But there was no pattern, numbers were seemingly picked out at random, and with many gaps. Sometimes a cab got a different designation for relatively minor reasons, for example, the S24-39 models all used the same cab shell and some were nigh on impossible to tell apart as it was dependent on
