Claiming back an original number is quite a rigorous process.
You need a document which connects the registration number to the frame number stamped on the bike.
That means an old log book or a certified copy of an entry from the old motor tax records which also links the frame number and registration number - most of the surviving old records dont show the frame number or possibly some pre 1983 document which links the frame and registration.
DVLA only consider applications which come via the V765 scheme.
https://www.gov.uk/vehicle-registration/old-vehiclesThe reverse MY series of numbers is a Middlesex series used between February 1961 and July 1962.
It is worth researching with the Kithead Trust whether they hold any of these reversed Middlesex records.