RAF Operations Clock

You are forgetting the US Army Air Force sector clocks which were also used by the RAF in the ATAF, made by Seth Thomas with a face having five-minute segments using red, yellow and blue but also green, yellow, red and blue 15 minute colour segments.

MM is that your living room clock? Or the one at St Mawgan.

Just noticed you may have the Mks wrong, according to the official AHB official history on Raid & Reporting etc (pub 1952) the earliest pre-WWII version had ten minute segments which were used in the first filter room at Bawdsey Manor in July 1937. After the Air Defence Exercises of that year, ten minute segments were reduced to five, as the filter room table was quickly congested as the tracks were remaining on the table too long. It is very unlikely though that you will ever see a ten-minute version as the originals were either re-faced or over-painted with 5 minute segments.