No6 isnt the number of the adaptor, its the number of the caliper,much like that No3 your holding.Caliper is in 2 halves ,the half with the number on it is the side that has varying spacing.
the no3 you hold fits on IS mount 51mm ,on the rear a 165mm rotor is used ,the same no3 on the front allows for a 185mm rotor[rotor sizes without +20mm adaptor]