Dave,
It is possible to trace the officer with his clothes number as they are unique to each officer. This is not his force number however. Sgt's wore their stripes on the sleeve of their tunic but their number was displayed on their shoulder. Later the chevrons (stripes) and number were both displayed on the shoulder when wearing jumpers or short coats. Its true that officers from the rank of Inspector upwards did display a clothes number.
It was not really necessary are there were only a few of them and not much chance of getting tunics etc mixed up. There rank of course was displayed on the shoulder.
The R was necessary as reserve police were numbered numerically like regular officers so without the R there would be duplication of every number. Nowadays PSNI officers were their clothes number in the same way as the RUC did and they display their name, unit or station but not their force number.
Hope this helps.
Regards
R