Hi I think there are 3 different types of rear brake levers, the 500 type is tubular and almost straight, also fits the early 550s (k models like the 500 US market), then the 550 F which had a flat metal construction to clear the footrest and 4/1 exhaust. The 550 k3 was different again, tubular but with a deep curve. I have a 500 and also a 550 f lever both will need rechroming.
cheers, Mick