Charlie it's a complete mystery to me, the only one I've seen and not listed anywhere as far as I know. It's an old one for sure. Tang stamp reads simply Plymouth Knife Co so I'm not sure if it originated in Plymouth, Mass or Plymouth Hollow in Connecticut. Plymouth Hollow was the home of the American Knife Co (later Thomaston).
I like that pic and have another at a slightly different angle that looks the same. I'm not sure how I did it It's called teal and that pic looks like teal, but its the same knife in my avatar and usually looks more green than teal: