Rich, the cannons on the 1/5.3 scale Taylor design are just slightly under 7/8" at their widest point. I would say that 7/8" on your 1/5 Platt bird is in the ball park. The blister on the top of the Mk.XII cowl is unique to that variant. I'm not certain but I think the blister is related to the Coffman cartridge type starter.