Even though the swingarm is in the printed & dated 1977 parts catalogue (which I have a copy of) and even though there are many other aftermarket alloy swingarms passed as legal for the '77 RM125, some officials will not pass it as they say there is not enough proof.
I have seen a pic from someone on this forum (can't remember who) of Anthony Gunter, taken in 77, using the optional arm on a B model. You will have to dig up more proof; this will make optional B arm accepted.