Doug Nye's Cooper Cars describes the T14 as " Cooper-MG production sports" from 1950 and the T21 as "Latest Cooper-MG sports" from 1952. He doesn't include any chassis records, presumably because they have not survived.
He gives very little history of the Cooper-MGs but he does say that when Lionel Leonard sold KOY 500 to Horace Gould, the latter fitted Mark 1 Cooper-Bristol (T20) suspension and twin leading shoe brakes. This suggests KOY 500 started life as a T14 and was uprated to T21 specification.
He also says that many of the Cooper-MGs were kit-built with considerable variation.
Martin Krejki lists Leonard as racing JOY 500 in 1951 and KOY 500 in 1952 in 'British National' and 'British International' racing. This suggests that JOY 500 was a T14 and that KOY 500 being a 1952 car was a T21.
At the time, the 'OY' on the number plate denoted the registration authority, in this case Croydon. KOY regisrations would have followed on from the JOY ones. I don't know how long it would take to use the numbers as it depends on the number of cars registered at the Croydon office.
Remember that the early Cooper type numbers are somewhat unofficial, having been introduced after the event by journalists (DSJ?) to identify the different models. I suspect that one model evolved by stages into the other and this particular car could be either.
Ralph, I expect you know all this anyway. It will take a Cooper expert (which I am not) to answer your question.
Edited by D-Type, 11 November 2017 - 18:20.