Couldn't make the photo-link work for some reason, but according to the Lucas parts lists - those interminably long and complex things that take ages to plough through and cross-reference - the right mag is K2F 42263D/E, which is 'Anti-clock, Spring, left-hand' in the Lucas terminology. There are loads of mags very similar, so the actual one on your bike may well not bear that number after all these years of course. In any case, anyone can stick any plate on any mag body . . .
All the bits are laboriously listed in the Lucas lists - one source for all of which is
http://www.brightsparkmagnetos.com/library/Parts%20lists/Lucas%20motor%20cycle%20parts%201936-57/G09-23%20Magnetos%20and%20ATDs.pdf It's a guaranteed way to accelerate the need for strong spectacles and go cross-eyed, but needs must from time to time.
From what I can see then, and if I read it right with fading sight, the AR cable would be on the cylinder ('left') side of the cb end housing on a Star Twin looking at it from the cb end. Indeed, to have the cable slack in the advanced position on an anti-clock instrument, it would need to be on the forward side of the housing.
I have some of the same cable-routing problem on my s/arm A10, due to the float chamber of the Monobloc getting in the way. But that's my fault, as that bike should really have the ATD that's in the box of bits put back, and a fixed ringcam.
good luck
Bill
(with my Brightspark anorak hat on)
