The website
psv badges is probably worth a visit for more.
this page details the regional letters, although I'd add that there was (in the 1930 act which established the Traffic Commissioners) a Southern Traffic Area, which was abolished before the 1939 War - I can't recall seeing any J badges (from this area) on the collectors' market but it's fair to assume some must have been issued.
The traffic areas did not exactly match county boundaries, e.g. Chesterfield was in the Yorkshire area and Buxton in the North Western area. (I'm not entirely sure now whether what badge / licence you got issued depended on your home address, depot address, or company's head office address.)
Prior to the 1930 Act, bus drivers and conductors were licensed under hackney carriage law by local authorities - some do appear for sale now and then, along with many post-1930 taxi driver badges being described for sale as bus driver licence badges on the collectors' market.
Also, the T for tram and trolleybus on a PSV style badge was a London thing (elsewhere, local authorities were the licensing authority for tram / trolleybus crews and had their own standards for badges etc, some operators' crews wore individual numbers on their collars like the police used to, others were issued cap badges with an individual number.) The
Tramway Badges and Buttons website has a lot more.
Some operators' tram or trolleybus crews (particularly conductors) might also hold PSV licences / badges so they could work buses as well. Blackpool at one time had the concept of 'three badge men' who could drive trams or buses and also had a PSV conductor badge.
Also, my understanding is that the 'NN' badges that are now in circulation (I have only ever seen NN conductor badges in the 122xxx range) were made in error and never officially issued, I have heard a few alternative explanations for their use, although I have never come across any evidence for any of these explanations, and never saw a LT conductor wearing one. Likewise there are theories out there why the Metropolitan Traffic Area badges just got a single N and every other area had the letter repeated but I'm not sure anyone has established a definitive reason for it.
Broadly, PSV conductor licensing (and therefore issue of badges by the traffic commissioners) ended c. 1980 / 1, and PSV driver badges ceased in 1991 as part of the changes from PSV to PCV licensing around that time.
Towards the end of driver badge issues, some drivers got badges from the 'wrong' area - again speculation, but it seems fairly likely that some areas ran out of badges and others had plenty left, and this was the most economical way of dealing with it - a wave of London drivers got 'LL' badges, and the last few badged drivers in the West Midlands area got 'CC' badges.
After conductor licensing ended, some of the operators who still had conductors (or possibly those who were still taking on new conductors) did choose to issue their own badges. London Transport issued green edged conductor badges which may have continued the 'N' number sequence (or may have started again at a chosen 6 digit number) although without a letter (these were only issued to new starters, with existing conductors keeping their N badges.)
Blackpool Transport issued green edged BT 1001+ badges for bus / tram conductors and also blue edged 'MTM 001+' badges to tram 'motormen' - the latter of course including some women by the 1980s. Many of the staff who were bus conductors in winter became tram drivers in summer, and seasonal conductors worked buses as well as trams, with a relatively small number of full time tram drivers.
I'm aware that South Yorkshire PTE issued green edged conductor badges with the SY logo and a 3 or 4 digit number, and I have seen National Bus Companies' conductor badges (Ribble and Devon General are the only ones I can remember seeing examples of, but there may well have been others) which were on a mid blue background (similar to the NBC uniform blue colour.)
After driver badging was discontinued, a wide range of PSV type badges were issued by many operators, some included the traditional red edges but often with a company logo in place of the letters, some kept the badge shape but followed the company's current corporate colours, and there was the strange choice of green edged driver badges by Nottingham and Maidstone and District. Most did not keep it up after the initial issue ran out.
I understand that badges are still issued to drivers on TFL bus services, usually the letters indicate the operator and / or garage, although wearing of these badges in any visible way does not seem to be enforced.
