1967 Sea Flight "K".
After 3 years, I offer vindication for Geoff and Ken. Subject watch is a 1967 Sea Flight, but not a "K" variant. Here is a 1967 "K" with string tag that is a perfect match to ad. This tagged watch also has a stainless steel back with threaded ring.
Nice original find. I'll remove the variant and leave it as a generic Sea Flight.