"O.S. .46AX's owner's manual suggests a propeller range of 10.5X6 to 12X7"

Real life operating parameters often conflict with "manufacturer's recommendations". I don't know how manufacturers determine prop sizes, but I suspect it's kinda like auto gas mileage figures on the window sticker - optimum conditions, possibly even a specially set-up vehicle.

A 12 x 7 seems a little large for this engine, IMO, and I've seen .46 engines being run with 9 x 8s in speed planes. It all depends on the application.

Dr.1