This pic shows an NOS typical Model A and B style B-6017-A1 side timing cover (was A-6017-A1). This is a machined cast iron cover.
Note that this cover uses one long and one short side bolt.
The pic above is the Ford-Köln B-6017-A1 side cover found on some German G28T Model BB engines. It is machined cast iron the same as US models, but also has the F-K logo impression. This particular cover was from a 1952 vintage G28T engine from a G388T Ruhr truck.
The pic above is the unusual A-6017-A2 die cast magnesium side cover found on some 1931-32 Model A and B engines.
This cover uses two short bolts, rather than both a long and a short bolt like the common cast iron cover. Also, the bolts use a flat brass washer under the head when used with the magnesium cover.
This magnesium cover was released in January 1931 and obsoleted in November 1932 per the drawing records at the Benson Ford Research Center. The drawing is marked "Experimental Manufacturing".
It may be that this was a limited production part Ford was evaluating for potential cost savings. Though magnesium is a premium material cost over iron, the magnesium cover is die cast and requires no foundry operations and no secondary machining operations and generates very little casting or machining scrap.
Ford also produced some experimental Model B die cast magnesium valve covers, as described in a related link listed below.