Somebody is having a laugh, either at the mint in 1965, or post mint in a shed somewhere. The obverse is the 1953 one with BRITT OMN. Where is the coin? If on ebay, then someone is probably pulling a fast one. If you have it, then you should be able to check for a join where one side has been dropped into the other (probably the obv looking at the beading). If on ebay for a quid, then buy it and satisfy your curiosity.
It aint right whatever