Saving Old Seagulls

have acquired an SJP...very clearly stamped
BUT

ID is SJP 478 H6 ?????

Can't be Aug 1966 with that No ???????
can't be 1955-56 with the H6 bit ???

Don't be fooled by the British Seagull secret code. It was never designed to be easy. They started the numbering off aqgain, so whilst completely illogical, your date code is quite correct.

So August 1966 it is...