I've just been having a good look at how it's made. Until your post I hadn't considered how it may have been put together, just how impossible it looked to take apart.

I think you are right that the single cable must have been crimped in last. I wondered if the splitter case might have a seam in it, showing that it had been 'wrapped' around the assembled cables but it's definitely one piece and looks like its cast out of brass. If it had been fitted last they would have had to chrome it in place. I think they must have crimped the double cables together and then used a, perhaps specially made, long-nosed crimper of some kind to crimp the single cable between them inside the casing. The lower end of the crimp is 10mm deep within the case with the single cable adjuster as far in at the top as it will go.

Apart from all that, it's a very well made piece of kit. The splitter case and telescopic spring cases are chrome plated brass, the threads, cable end ferrules and adjusters are brass and the nuts and wires steel. Although the cable casings are cracked and frayed here and there the wires are good and the springs strong. Good for another 60 years, I wouldn't wonder.

Something has just occurred to me, that perhaps back in the day dealers were equipped to un-crimp the cables, replace a part and re-crimp it all together again. I can also imagine that with it all being so well made, over-engineered even, it never needed doing.