Just checked the fleet list, and you're right Percy Main don't have any at the moment. When it's dark outside and I'm in a passing bus, it's very hard to see fleet numbers, but they were definitely B7's for sure! The first two journeys from Sunderland appear to run light from Percy Main depot before service. Perhaps the times I've seen B7's on the 9A, they have been supplied by Deptford instead; if Percy Main for whatever reason were unable to get a vehicle down in time.

The next time I see a B7 on the 9A, I'll try my best to get a fleet number just to see if that's the case.