I have found that the Smithsonian have an example of an A. T. Reynolds made MK V Altimeter.
Simple Altimeter, British | Collections Search Center, Smithsonian Institution
The serial number that I can read is 355/?U.
I would like to know what the missing letter is from the serial number. If anyone has an A. T. Reynolds made instrument, could you please advise what its serial number is, so that I can solve this mystery.