There can’t be many two-jaw clutches out there that look exactly like this, but it was the first time I had ever come across this pencil so hopefully it’s not too obvious.
Based on the color, and being “not Japanese”, and being a clutch pencil, I’m guessing a Mars 780 but that’s a guess because I’ve never seen one in person, nor its insides.
To me, the plastic looks too new for 1960’s… even with a well kept specimen. But with the strange 2-prong clutch, I’m thinking 1970’s. Maybe… Sheaffer?
Oh wait… “wrong side of the Atlantic” in response to Scripto., then it can’t be USA.
You’re getting warmer, longitude-wise. I would guess it dates from the late 1960s or early 70s, but can’t really be sure because I have never seen it in print.