How do you date a dunhill lighter?

Anything with a patent number or serial number is in the 70's or later. Anything before the 70's may just say PATENTED and the serial number is inside the casing. It is not recommended to disassemble yourself but if you keep the bank in tact and just unscrew it from the case, you're fine. If it doesn't say PATENTED but still says made in Switzerland and the Dunhill logo, most likely from before the 40's.