I measured up the light, its 160 x 100. and yes I see that the MC 18 is smaller than the 16's.

The bike has a plate from Aiken's MC and says its a MC18, 1989. Doesn't say when it was imported here (Aust) however. So they must have done the modification to get it registered, doesn't look factory but it's not a bodge either.

So I guess your right with the answer.

Thanks Neil