Yes it's rubber. Laura from ESPO says the inner metal sleeve of the bushing should be clamped tight by the bracket; that agrees with your statement. I suspect I could get a couple more turns on the bracket nut. The rubber is not supposed to slide relative to the metal parts of the bushing but instead it should twist as the spring moves. (I guess that's how bushings are always supposed to work?) Finally, she said never use WD-40 on rubber because any kind of grease breaks it down. I guess I should have known that.