There was a revision(additional stand off points) to Masterbox 5 that added support for E-Atx/SSI-EEB boards. USB 3.2 Gen 1 Type C is 5Gbps USB 3.0, also manual confirms there are just 2x 20 pin USB cables for front panel connectors with Sata power plug.
CoolerMaster has posted NR600P on their website. Its using really old MasterBox 5 chassis and even the Type-C port is only 5Gbps USB 3.0 with all ports and SD reader connected via 2x 20pin USB 3.0 headers.
B550 seems to be more expensive than equivalent X570 SKUs(atleast in India and in case of Asus who has officially announced their pricing). That thunderbolt is why said a step forward, in general GB has dropped internal type-C header from all their B550 boards which is very odd. I remember some...
Look at last photo(package contents) it looks similar to all the recent Asus HEDT boards(X299 refresh, C621 and X399) which had solid slabs of aluminium with fans stuck on top and bare minimum of fins.