BB
Kelvin is a good make & from my long passed techy days may have been fitted to Vulcan, JP or Meteor a/c (Beagle maybe able to answer that). If the serial number is painted on (as you state) then it is probably a unit serial number that is pretty meaningless now. If there is a data plate bonded (or mor often riveted) onto the case that may provide a part number, it may also be engraved or stamped (and painted over black) onto the dial face, look carefully here. Good Luck.
R2