mars bar for the question. Some think it is to do with keeping stub 6 away from wooden parts of frame, more likely that the radiator did not leave enough room on the inboard side of the nacelle, on earlier variants, so 5 and 6 stubs were combined. Some variants had 5 stubs both inboard and outboard, for neatness. later variants had a longer nacelle - and hence room for 6 stubs per side.
These are the guys who did the work. True artisans. Avspecs - Past Projects not forgetting the genius who has the moulds to make more Mosquito Aircraft Restoration. This shows the rear two cylinders exit through the same exhaust stub. 0763 And this Love the "popping and farting" reference
Wonderful aircraft and what an epic pair of engines Thanks for posting. The last "Mosquito" derivative was the DH Hornet that came into service right at the end of WW2 with an amazing top speed of 472 mph. Those Merlins were knocking out about 1600 hp each in what was a pretty light airframe.