Glad to see the RM125B is one of the most discussed bikes here (swingarm and forks).
The optional RM125B aluminum swingarm is legal here in U.S. VMX, but we had debates about it for a year before it was given the seal of approval. It is not a very good aluminum swingarm (more bling really). It is heavy and there are a lot better legal options. I love the year cutoff you guys use for your rules, but I don't think the world would end if someone ran the arm in OZ and I'm one of those tight asses about rules.
Here were my three arguments I used for making the arm legal here:
1. Built by the factory for the B model
2. Available to the public from your local Suzuki dealer
3. No advantage over legal aftermarket swingarms
Seems like the mountain (#2) to get over for making this arm legal is a tough one, unless you can find a magazine article testing one or an advertisement about one, prior to Feb 1978 issues (magazines were published a month ahead, as they are now).