Generally TA regts didn't use A-D bty letters, they were all numbered (with the exception of some TA RHA). As far as I know, and ignoring RHA, only a handful of regular regts used letters and a handful of regts raised outside UK as an interim measure until they were given bty numbers.
However, 73 wasn't TA. Basically the first 5 TA atk regts were formed by converting TA fd regts in 1938/9, the next 5 by converrting TA infanty bns, the next 10 by doubling this 10. This gives us 51 - 70 regts. After that they were War Formed regts. I think bty numbers were sequential starting in 51 regt (the handful of regular atk regts used reg numbers). The issue is to find the starting number in 51 regt. However, the problem is that there was some fluctuation between 3 and 4 bty regts which have screwed up the sequence.
The starting number was 201, and the first 10 regts seem to have had 4 btys. 69 Atk regt had 273-6 btys.ers.