Seeing that pic of the C-123... in the summer of '77 at my school a few miles south of Guildford, I was playing cricket and a high wing, twin radial engined transport droned NE towards London at about the height and on the same route that airliners approaching the Ockham hold followed. Without the aid of binoculars and mindful of the cricket ball being hoicked around the pitch, my glances identified it as a C-123 - pretty sure it wasn't a Pembroke or a Sea Prince. Can't think of anything else it might have been, but did the USAF have any in Europe as late as that? Might have been civvy one being ferried somewhere interesting I suppose.
Occasionally saw USN C-131s or C-118s on the same route as I recall, inbound Northolt.