I was told this might be an rc swingarm. I will check when home for the holidays. Not sure about the brace but will look.
Bike was raced and daily driven in the 80's, 90's by p.o. He said he would ride it to track with struts and wheelie bar strapped to the back rest. 890 cc with R.C. rods,
btw, not me in pic. My bf Thad.
HB