Congrats on your find...and an AT Pro with a 5x8 coil is an excellent choice! My detector and coil of choice. Now, be careful about cleaning silver coins. What I mean is...you'll get a variety of opinions on cleaning silver coins; some will say just use running water, while others will use a Dremel. A "cleaned" coin is generally worth less when selling it on the open market, but shiny ones can look cool. I only say this since your Merc looks polished with silver cleaner (man, I do know how to pop party balloons). Really, WTG.
aj