Cables from the lever right to the operating arms on the drums on Robins & Kittens, 1973 to 1983, according to my Haynes manual. The later Rialtos would be cable too, so I assume yours is an earlier Regal axle?
You will have to bear with me on that Andy cos I am not sure which it is, does this help?
Virago 750 Trike
I thought the regal axle was the 3 piece jobby that is good for chain drive conversions? I have one of those as well for Caz's trike when I get back on to it. Can get some better pics of the diff if you can't make it out from this but I thought this was a robin?