I have a watercolour of an English scene done by my grandmother in 1929. It was framed by T.N. Ahuja & Co in Rangoon. The back appears to be lead! Is there something exciting lurking behind or was the lead just to protect the painting?
It is probable that with glass and a metal protective back, a well-constructed frame would have been watertight. Perhaps this was intended to protect the contents on a sea voyage home.
A frame 16"x15" is more than large enough to hold documents on foolscap paper under the lead cover. Without the oversized(?) 3" mat, the picture itself would be smaller than foolscap. Whether this convinces you that there could be something other than the picture inside depends on how curious you are already.
If you do decide to investigate further, do consult a dealer about the possible rarity of the object. You would not want to ruin an example of a little-known framing technique for nothing!
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.