That is a leg from a Denys Fisher Muton action figure, released in the late seventies. Muton had several "sub-form" costumes that he "transformed" into. The leg you have is from the "Torg" sub-form.
I used to sell accessories for just about everything in my old collector's shop in Croydon and the one I had later in Portobello Road market.
The leg you have is quite rare. You'll get a few quid for it on Ebay, for sure.