(sorry not very clear picture)

don't know if this is original or not? if its what i think it is my mini used to have something similar, like a little valve that can be shut and opened with a cable or something, could be this not opening and letting hot water through?
i haven't done the obvious yet and got it up to temp and felt each pipe (in/out) etc
but thought id just ask if anyone knew what it was?
Cheers
Mike