Anyways, I bought an Autotecnica boss & wheel while I was in Melbourne, and it didn't fit, either. I went and saw the guys at Autotecnica, who could not make it fit no hard they tried. In the end, they researched it and sourced a HKB boss for me directly from Japan, which fits. Just. The box is marked "323 Early Mighty Boy". The splines match, but the shaft still won't slide on - will need machining very slightly.
So, on it goes anyway (probably not very smart), and promply ruined my switchgear. Another set of switchgear and some minor adjustments, and still things went clunk. It's still there, but my indicators don't self-cancel anymore.
Thing is, the pins are exactly the same offset and length now, just the boss doesn't sit as far down the spline as it should. I didn't think it would be such a big problem. I will get someone to machine the hole a bit, and hopefully that will help.
If anyone has any more useful input, that would be great - those switchgear sets are ruining me

Thanks,
Sean