It's not clear to me why a non tower rated controller would be rostered for the early turn in CBR
Quite simply, the other controller woud have most likely been rated one of two ways:
1. SMC/COORD/ACD - ie, rated as a Ground controller, as well as a few other things, but not tower rated. This is quite normal at many aerodromes. The first few hours would involve him/her doing these functions, whilst the other "Senior Tower" does the Aerodrome Control. Or;
2. Rated in all positions, except OCA - OCA (Operational Command Authority) is sort of like being the Captain of an aircraft. A good FO can take off, fly and land an aircraft all by himself. In fact the Captain can leave the flight deck and the FO can fly just fine, however, the Captain needs to be in the aircraft to make it all legal. Why not put 2 Captains on every aircraft? Because there is no need. A Captain and a FO do just fine. Same reason why there might not be 2 OCA's rostered on every ATC shift. There needs to be one to make it all "legal like", but the other guy can do all of the other jobs him/herself.
I can only assume that is what was meant by "Senior Controller". Hope I helped.