The cateye rear one mounts to the saddle itself - I'm not aware of one which mounts to the rails. https://tinyurl.com/y9q86e4g
But it has to be a 2016 (or later) saddle - with a threaded insert moulded into the underneath of the saddle.
That’s the one. I haven’t used it in months so couldn’t remember if it mounted to the rails or the saddle, and was too lazy to get up and walk 4 steps to look.
The cateye rear one mounts to the saddle itself - I'm not aware of one which mounts to the rails.
https://tinyurl.com/y9q86e4g
But it has to be a 2016 (or later) saddle - with a threaded insert moulded into the underneath of the saddle.