Why is Greenley Called C?
The story behind MI6's first code name.

‘C’ was the code name for RNR Commander later Captain, and later Sir Mansfield Cumming. He was the first real head of the secret service for many years and the code name has stuck.

Spy writer Ian Fleming used the code name M as a tip of the hat to Sir Stewart Menzies, the head of MI6 during his time in harness during WW 2. Incidentally, you will notice a box of green ink on Greenley’s desk.

This is a joke.

Cumming always wrote with green ink. Doubtless someone on the Sandbaggers set knew this and put a box of Quink brand ink on Greenly’s desk.Since the Sandbaggers aired, Parker bought Quink and absorbed the brand into its line.

The green C has even been made an official part of SIS lore. It's in the service's crest. The C surrounds a brain symbolizing intelligence. The crest is accompanied by the motto semper occultus, or 'always unknown.'