A very nice find!! I have a number of this kind of strike from various casinos (although none quite as impressive as yours).
These are created when excess silver is squeezed out over the brass, usually associated with an off-center planchet.
Handle carefully as the thin sheet of excess silver can break off leaving a dent in the brass where the silver used to be.
Send photos to Jim Quinn "webmaster@silverstrikers.com" if you wish to include your unusual strike in the Club catalog.