For what it's worth, in "THE ILLUSTRATED GUIDE TO MILITARY MOTORCYCLES" the restored WW 2 AJS, Ariel, BSA, Matchless, Norton, Royal Enfield and Triumph all had bare aluminum engine cases.
The right side cam covers on these were bare aluminum.
The primary cases on the left side of the AJS, Ariel, Matchless, Norton and Royal Enfield were painted OD.
The engine case on the Volocette is bare aluminum but the left side primary and right side cam gear cases are OD.
The tire rims and spokes on all of them are OD.