That is a sweet bike. I'm not sure about that exhaust, but a friend's father has a BSA Rickman with what we think is a Rickman exhaust. I think his is going up on the auction block soon though. Probably take it to Monterey. Sweet bike. 850 (or larger) cc BSA engine that started out as a 650, cerianni front end, dual drum front brakes with two leading shoes per drum (actually stops), full Rickman body with three tanks, shouldered Akront rims, magneto ignition. The list goes on and on. Just a beautiful bike. Rickman really made some amazing machines.
By the way, the exhaust on the BSA Rickman is only headers. He tried fabricating a muffler for it (he built it while working at Sandy Cozman's shop in SF), but he couldn't figure out a way to keep the muffler from blowing off. I guess some engines just don't want to be muffled. I've never seen a Rickman muffler though.
Camelman