what prevents the tabs from spinning around the bolt? and what did you tap the bolt into?
Keen eye.
The previous owner had started to build a bobber. So fortuitous to me, he welded and tapped two substantial mounting points to bolt on a bobber solo seat, which I have since sold.
I simply repurposed the mounting points and used them for the Tuffside seat. Tuffside seats have studs attached to the bottom of the seat pan and will be secured to the tabs with the provided washers, lock washers and wingnuts.
I don't think the tabs can spin since they are mounted securely at both the frame and the seat pan. The Tuffside seat pan was also molded specifically for the cb500/550 frame so it sits incredibly snug on and over the sides of the frame.
I originally was going to use the rear mounting studs for the Tuffside seat and had them positioned for the first crossbar you see below. But since I want to mount a rear fender, that made access a little more tight than I cared for. Easy access in the current position and just as secure.
Here is a picture of one of the mounting points from below the frame and both points on top of the frame.