It these were consecutive and included all letters, then this fantasy gothic lettering (as I call it) would have started in 1948
The rings - earliest top right, latest bottom left

R - 1965

S - 1966

T - 1967

W - 1970

x - 1971
Note the style of the letters change to a more roman look

v - 1972
The exception - this should be y but really look like it always was a v - maybe it's just really badly struck or could it be old stock from 1969 the NCAU used in 1971 - but then surely it should be in that fantasy gothic style

z- 1973

A - 1974
This is another Roman style

Many thanks Frank