Aah, well, we pulled it to bits today.
For the record here, the two bearings were 6203TB.P63, and 6206TB.P63.
Single row, deep groove ball bearing.
6203 is 17mm ID, 40mm OD, 12mm thick.
6206 is 30mm ID, 62mm OD, 16mm thick.
They are a very high speed version of the common 6203 2RS designation, but the 2RS ones have a max speed far too slow for the router spindle. (like 7500 RPM?)
The balls look like they're steel, though we thought we were told they'd had ceramic ones fitted just before we bought it. Evidently not!