I have had more than a few genuine WW2 WD M.20 outer chaincase covers over the years, some with damage and others appearing good. The general comment from those whom I have asked to repair holes in some has been the metal is extremely poor quality steel. I had one in particular that looked good but discovered it was porous with a constant film of oil on the outside of the chaincase.

I suppose that is how it was at the time - if it did not require top quality material you did not get it.