Dan, I believe these are the brandings based on my sightings on the first day in service (I'm not sure about 4644 and 4645 but I think they both have branding for both the X26/X27 and X66).

4643 - MAX branding for both the X26/X27 and X66
4644 - unsure (I think it has branding for both the X26/X27 and X66)
4645 - unsure (I think it has branding for both the X26/X27 and X66)
4647 - MAX X26/X27
4648 - MAX X26/X27
4649 - MAX X66
4651 - MAX X66
4654 - unsure
4655 - MAX X66
4656 - MAX X66
4657 - MAX X66

I think 4646 is at Darlington but have no idea whether it has combined branding or X26/X27 branding - fleet list says it is still at Belmont in original livery.