In The Good Old Days, every "Fun With Chemistry" book included the "Silver Penny" experiment. You'd take a copper "penny", soak it in lemon juice or scrub it with abrasive to get the surface clean, and then rub it with mercury. The mercury amalgamates (mixes) with the copper, and the resulting alloy looks like silver. And hardly anyone ended up with permanent brain damage. But just try and get mercury now...
It could also be that somebody working for a plating company put it in the chroming tank. Before you chrome a car bumper you must first copper plate it. You can still get mercury if you try hard enough (old thermometers, thermostats, etc.). However, posession of a large enough amount without the proper papers is a felony. Many years ago someone was busted for dealing drugs. During the raid, the officers noticed a large ash tray with a pool of mercury in it. The dealer not only was charged with distribution but also felony posession of mercury.
Well, that's new. It looks like this is a matter of state law. The first hit I found was for Oklahoma, where it's a felony to possess more than one pound of mercury without "valid written evidence of his title to such mercury", and with defense allowed if you're a "bona fide miner or processor of mercury" or if the mercury is "an integral part of a tool, instrument, or device used for a beneficial purpose". http://public.esquireempire.com/Pos...Mercury-Oklahoma-Felony-Okla-Stat-tit-21-1726 Most of the stuff I've found for North Carolina pertains to dealing with mercury switches in scrapped vehicles, and getting the stuff out of schools. When, er, if I had bought a couple pounds off eBay years ago, I didn't think about new laws lacking grandfather clauses. Sigh. It's a shame the stuff is so insidious and persistent; liquid metals are all sorts of fun, especially when they're as dense and standoffish as mercury. (Gallium-indium is liquid at room temperature, but it costs dollars a gram, it sticks to everything, it gets a nasty oxide film, and it's less dense than iron.) Might as well black out all the experiments dealing with it, along with the ones calling for carbon tetrachloride, iodine, phosphorus...
I've never heard of any laws pertaining to the legal possession of mercury. OK did some checking I can't find any legal restrictions on possession except in OK and NM where possession of over a pound of mercury without bill of sale is a felony. Under a pound seems to be OK. No special license appears to be needed, just the bill of sale showing where and when you bought it and from whom.
I remember back in the 1950's that there were a number of keychain manufacturers that used silver or gold plated Lincoln cents encased in a bezel. This coin appears to have something going on with the rim, and looks similar to those coins. That would be my guess.