The Tin

While fossicking around the hard rubbish piles in Melbourne's Eastern suburbs, I spotted a metal cigarette tin that had been overpainted to resemble some sort of flag. Curious. Picking it up I could feel there was something inside the tin. By now my spidey senses were tingling. Just to prolong that feeling I thought I'd open it when I got home.

Later in the day, upon opening the tin I found.....THIS!

Ok, that's a cash box envelope, but more importantly, that biro pen scrawl seems to indicate a National Australia Bank box number. Could it be? Looking inside the envelope  I found this.....

OMG it's a KEY!  I have no idea what to do about this. Please advise.