In South Yorkshire, the services before deregulation used to be something like
1 - 99 - Sheffield services
100 - 149 - Rotherham services
150 199 - some Doncaster services
200 - 299 - "other" services (e.g. Sheffield - Penistone was the 239, Sheffield - Doncaster was the 277/278, lots of services in the Dearne Valley that crossed between the boundaries of Barnsley/ Rotherham/ Doncaster)
300 - 399 - Barnsley services
400 - 499 other Doncaster services
(with a few exceptions where things didnt neatly fit boundaries, or doubled numbers where an out of town service duplicated a local one, like the East Midlands Motor Services from Derbyshire)
For whatever reason, the lower numbered Sheffield services weren't particularly frequent (the 2 was an hourly outer circle, the 3 and 4 were hourly, the 5 was a Sunday only service with a handful of journeys...), I cant remember a 6/7, the 8 and 9 were hourly inner circle - the flagship frequent routes had numbers in the fifties/ sixties/ seventies/ eighties. So that when First introduced their initial Overground, I think the second lowest numbered service was the 41 (could be wrong the 33/34 were grey lines on the map as their combined section was every ten minutes I think the 22 was the only proper Overground route below 41).
Then, the three main towns lost their three digit numbers e.g. that the 137 in Rotherham became the 37, the 455 in Doncaster became the 55 (but some longer distance services gained an X e.g. the X78 from Sheffield to Doncaster even though it wasnt limited stop). This contrasts with West Yorkshire, where the Bradford/ Huddersfield/ Halifax/ Wakefield services have generally retained three digit numbers.
Things are about to go full circle, with Rotherham town services regaining their three digit numbers. Not for any nostalgic attempt to appeal to the good old days, but a more cynical reason since Midland Road is closing, Rotherham services will be run from Olive Grove (Sheffield) or Leger Way (Doncaster), so having three digit numbers on Rotherham services avoids confusion on destination screens (e.g. drivers wont accidently confuse the 40/41 from Sheffield to Manor Top with the circular service round northern Rotherham as the Rotherham routes will go back to being the 140/141 that they were thirty years ago).
Oddly, although SYT (the dominant operator in Sheffield) didnt use low numbers for important routes, the competitors seemed shy of them too. SUT used 100/ 120/ 130 for their routes. Sheafline prefixed their services with a four (e.g. the 424 competed with the SYT 24 a practice that SYT retained when they took over Sheafline and used them as a low cost Magicbus equivalent competing with SYTs competitors e.g. the 472 introduced to compete with the Mike Groves 72). Only the short lived South Riding used single digit numbers on some routes (but mirrored SYT numbers where practical the 46 competing with the established 47 down one side of Gleadless Valley, with the 49 competing with the SYT 48 down the other side, the 85 competing with the existing 86 to Stannington).
Compare and contrast to Edinburgh, where LRT/ Lothian brought the numbers within the 1-50 (meaning an end to relatively long established cross city services like the 81, the 85/86, the 87). One quirk in Edinburgh was that LRT/ Lothian tended to avoid X services limited stop routes were generally put in the seventies as well as the aforementioned 85/86. So the 74 was the non-stop version of the 44, rather than the more obvious X44 that now provides a similar peak hour express.
In some of central belt Scotland (West Lothian, Falkirk, Stirlingshire?), the councils required tendered services to be in the two hundred series (i.e. the 216 was the early morning/ evening/ Sunday version of the 16). At least this would avoid confusion if a different operator won the tender to run the anti-social journeys. IIRC for a while commercial Dunfermline town services were seventy something but the eighty something for the tendered equivalents.
Dundee used to have some quirky numbers. The 28/29 ran from Douglas to Lochee, where they diverged to two termini. But all eastbound services were numbered 26 (I could have understood if they were all numbered 28 or all numbered 29, but they all had a different number). short journeys on cross city services used to have a different number to make it obvious that they didnt go any further (but this wasnt like the West Midlands habit of using a suffix or prefix to denote a part journey, this was a totally different number). I think that a short 17 was a 35 and a short 22 was a 30, but thats going back many years now
Didnt Belfast move to a literal clockface, with service numbers reflecting a clock face, in the way that the British A-road network radiates from London/ Edinburgh?
PS: Since weve had mention of the AD122, one of those other bus numbers that sticks in my memory was the Eastern Scottish/ Lowland Scottish X06 (which was the limited stop version of the 106, but easier for the driver to change to X06 on the old winding blind and leave the middle number untouched than it would have been to change to X6 are there any other services where driver convenience has taken priority?