lauriebe, the following link notes serial allocations were:
Mk II K2916 to K4130
Mk III K4156 to K6407
http://www.rafcommands.com/forum/sho...0-1937&p=81818
So seems to confirm your aircraft is a Mk.II, and I would assume since both the II and III had the same engine, the three blade was fitted to the II as well as the III, as in the photo of the III in my previous post. As to why? Easier on stores stocking one type? Trouble with the two blade in the tropical climate?
Photo of K2918 a Mk.II