I'm not sure why this half penny is stamped with the seal of York, but feel it must be a commemorative or celebratory piece.
Apparently, York is known as the "city of festivals" and by information supplied on their website, it certainly appears to be the case. Would this be a commemoration of a special city event?
The only other thing that came to mind was that 1966 is the 900th anniversary of the Norman conquest of 1066. I don't know if this has a special meaning to the city or not, but I couldn't come up with anything else. BTW, the obverse is un-stamped.
Any explanation that you can think of?
Many thanks,
Bruce