I have come across a 1983 20 pence coin, it seems to have been struck on either copper or bronze, there is also a fletch of "silver" on one face. its also thinner than a regular 20p coin. how do I verify whether this is indeed a genuine coin?
It is most likely to have been buried in the soil for a while: that usually turns cupro-nickel coins brown. If it weighs the same as a normal coin, then it will just have this environmental damage. I find dozens of brown 20p coins with my metal detector! The thinness could be due to corrosion in acidic soil or on a beach.
to the Forum! This is a great place to learn about coins!
That's corrosion, and definitely been buried in acidic soil for a considerable period of time. The bit of "silver" might simply be the original colour, maybe something covered that bit from being touched or the dirt around that area was scrubbed off.