Jon,
Given the crown config, Mick is almost certainly correct that this is a rehashed track iron.
Some more background perhaps...
In the UK in the 1970s there was indeed a fashion for TT bikes with track ends with a derailleur hanger because there was also an obsession with 'fag paper' clearances and weird frame geometries. Thus most conventional drop-outs at that time wouldn't permit the fitting / removal of the rear wheel without deflating the tubular!
So the set up you have on your Jackson was seen as the perfect solution for while - ignoring the xxxxxxx tangle / mess / greasy fingers you would get into trying the get the block under the chain and into the track slots... (so you can guess that I have been there - and done that one!)
Most self respecting testers ordering a new frame at that time would probably have gone for the then v.trendy sloping crown fork, and as yours is of a pattern which seems to be much earlier, Mick is therefore probably spot-on in his surmise (as usual!)
Ian Briggs (who rides much lazier angles and at lazier speeds now) LUTON UK.