The normal weight is 11.31 grams The coin look corroded so not sure if that accounts for the weight difference as well.
Have seen this before with coins found in or near water over long periods of time, the metal becomes lighter and brittle, will actually fatigue and break if bent too much. I think it may be some of the copper leaching out over time but I'm not a metallurgist so only speculating, however I have seen a lot in this condition from metal detector finds.
Thanks for your replies...metal detector find has been suggested by my LCS and I'm not sure why you would want to fake a florin either.
Because at the time it was worth 2 shillings. Several members of our coin club have large collections of fake predecimal coins. Modern fake coins are usually of high value coins and are aimed at coin collectors. Olden day fake coins were of common coins and made to be spent in shops. It would be worth more if it turned out to be a fake
Put me in the camp of "legit coin that's probably simply corroded (and possibly subsequently cleaned)"..
