18,
Dick may be right - it looks a bit too small to be an Avon - that engine just about filled all of the back end of a Hunter. It might be an RB-166 - an engine that started as a lift engine but was used as takeoff assist for Hot Weather operations on a Trident 2E (Think I've got the right model). The R stood for Rolls & the B for Barnoldswick (same as RB-211). IIRC it had the highest thrust/weight ratio of any turbojet in (then) commercial use.