Wasn't it the Ariel that changed from alloy to cast iron.
I believe the Ariel W/NG had a number of alloy parts changed to pressed steel during the war including the mag chain cover and the primary chain covers (inner and outer).
The Matchless G3L had the mag chain cover changed from alloy to cast iron and I think the Velocette MAF had some parts changed from alloy to cast iron possibly the gearbox end cover and the timing cover.
The Norton 16H had its tappet cover changed from alloy to pressed steel.
I think the only alloy item on an M.20 that was changed was the tappet cover which was cut from a steel sheet.
As a means of saving aluminum for more important projects, lots of bikes had parts made from steel instead, including the M20 tappet cover. Which as Keith stated was just a flat plate. Ron