S for solidus (Latin again). The pound symbol £ is an L. The names are way older than grammar schools.

The way to write and "pronounce" a sum is like this: £2 3/6: "two pounds, three and six".

There are lots more "rules": 1d is a penny, 2d is tupence and 3d is thrupence, 1/2d is a ha'penny (pronounced something like "hayp-knee"). So 2 1/2d = tuppence, ha'penny.

For good measure you also have terms such as: "ha'perth" (half penny worth). So: "For want of a ha'perth of tar, the ship was lost" which is what your ISO9000 system should be all about.