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!