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Thank you very much!Steve, I'm sure someone will follow with more details but the principle of the MG151 firing thru the prop hub is is fairly simple.
The cannon barrel is actually situated outside of the engine in the crotch of the Vee of the cylinders. Picture the DB 605 inverted (upside down) so the crankcase is on top and the gun barrel, outside of the block, below the crank. The crank has a gear on the end of it which engages another gear that is cocentric with the gun barrel. That gear is fixed to the prop shaft and the gun barrel goes through it. That's the basics.
I have a diagram of the arrangement somewhere and will see if I can scan it.