clamps style. speedway have then and some small pit bikes use the same. it just clamps on the tube section. so long as the chain is inside or running on the arm then its cool if not then it can rotate into the wheel no doubt......
on my champion framer i used 2 pieces of flat sheet then clamped it in with 4 bolts ( 2 either side of arm, the bottom 2 bolts also picked up on a Suzuki or YAmaha guide like 74 model ones. Seemed to work and stay in place.