It's hard to tell from your pic but patent # 2000752 was issued in May 1935 Graybill for a rodent trap, the document states the base is 5.6" long and 3" wide would something like that fit in that box?

You could have it X-rayed to try and find out what is in it with out opening it. Your local university may do it for you if you give them a cool story.

Very curious indeed =)