Mosquitoes with single stage engines, such as the B.IV, B.XX, FB.VI, FB.26 and FB.40 had 5 exhaust outlets with the rear pair of cylinders exhausting through a shared ejector exhaust.
Mosquitoes with 2 stage engines, such as the B.IX, PR.IX, B.XVI, PR.XVI and B.35 have individual ejector exhausts (6 per side of engine).
The reason is that the single stage engine's rear exhaust would interfere with the leading edge radiator, if not physically the exhaust would affect the radiator.
The 2 stage engines are several inches longer, so that problem doesn't exist.
There was at least one conversion, of a Canadian B.XX or B.25, from single stage to two stage Merlin that retained the original 5 outlet exhaust.