Your treasure does seem out of the ordinary.
The Dust Cover on the back of a painting is intended to protect the picture but it would usually be made from heavy paper not metal. See this discussion of methods
It might be that the Ahuja Co used non-standard methods for art framing because they seem to have been mainly a photographic studio.
The National Gallery of Australia has a small collection of photographic images dating from 1900 ascribed to the "studio" T.N. Ahuja and Co.
