Looks like Centrebus are a new contractor for West Midlands Rail Replacements as they also appeared back in May. This is probably due to Central Buses no longer being available
591 at Four Oaks
https://www.flickr.com/photos/128470593@N05/43717683011/in/dateposted/
773 at Sutton Coldfield
https://www.flickr.com/photos/128470593@N05/43001459784/in/dateposted/