I may be wrong, but i think that the bus which operates OS3 and OS4 (same vehicle) is the only one which doesn't have a duty which also includes work within the CAZ. Service 19 at the Bath end is within the CAZ, as is anything terminating in the city centre. I'd be amazed if Bath keep anything which isn't compliant as the penalties for a cock-up rather outweigh the potential savings.
Yes, I'd noticed this as well. Seems like the whole allocation for school services in Bath has changed since before lockdown 3.0. From what I can make out, these are the changes:
OS1/2 : Still worked in the same way - that is by the vehicle that does the two 37 trips a day (stayed the same in lockdown). Usually uses one of the "spare" Scanias (36822/4, plus 815 before it went to the IOW) due to the bridge at Bitton.
OS3/4 : Now has a dedicated vehicle that goes back to the depot between runs. As people have noticed this is usually one of 32543/5. Previously the morning bus went on to the 19 daytime allocation, while the afternoon bus was the same vehicle that did the morning OS5 runs. Both often used the two spare Bath Uni MMCs off the U1, but sometimes a B7/B9 could come out.
OS5 : Morning now uses a vehicle from the U5 allocation (the one that leaves the centre at xx:30 mins past the hour), so almost exclusively a blue Streetdeck. Afternoon uses a fresh bus out the depot, which also works the afternoon 20s run
(possibly this is also exempt from the CAZ?). Previously the OS5 used a dedicated school vehicle in the morning, while the afternoon run (along with the afternoon 20s) used one of the vehicles from the 171/2 cycle. This was often the duty done by 32291 before it left Bath.
3s : Now uses the same U5 Streetdeck as the OS5. Previously used a vehicle from the U1 allocation, which entered U1 service immediately after by doing a second lap round Oldfield Park. Probably confused any waiting students in the mornings!
20s : Morning uses a vehicle between trips on the 18/19 (usually a 12 plate B9), afternoon uses the same vehicle as the OS5 (this week either a B7 or Scania). Previously used another vehicle off the U1 allocation in the mornings, and then the shared OS5/171/172 bus in the afternoons.
So yes, in theory 1 (or maybe 2) B7s could still be used, but whether this actually happens remains to be seen!
On another note on allocations, I've noted a lot of buses swap between the 18/19/171/172/D1X during the day at the minute. I assume this may be to keep certain drivers on the same vehicle for multiple parts of their duty, but if anyone else knows why and can enlighten me, then feel free!