Great Britain Philatelic Society

I have a 6d 1797 cover to Cambridgeshire with a double ring receiving stamp on the back. It is dated Nov 24, 1797 and being double ring was sent in the evening and according to Hendy the letter to the left "F", denotes the inland branch at which is was received.

I have looked at my London Receivers book but could find anything there. Do you know where I can find what those letters correspond to?

In reading the London date codes booklet it seems to be a timestamp and indicates Morning.

Looking at a LPHG Notebook revision to BCC London, that mark is L7, evening duty sorting table F, a common (in black) mark from 1795-99. Codes A to F were used.