Serial 1 Rush/CTY Rear end looks like this: YDDIYJZFBJAENKAJE7GI4V2HZA.jpg (1500×1125) (cycleworld.com)

Big nut locates fore and aft for belt tension adjustment. There's Allen screw that actually loosens the hug to allow removal.

I haven't found a system yet for either 1 side spring mount or for my BOB trailer.

Thanks

SERIAL 1 Reply: Thank you for reaching out about regarding attachments to Serial 1 e-bikes.  
 
Liability prevents our team from making specific recommendations of trailers for use with our bikes that are intended to carry living things (human or otherwise).  There are unofficial methods however these can cause an undue burden on the motor and frame of the bike which would put the integrity of your bike and viability of your warranty in question. Serial 1 cannot guarantee the safety of any after-market trailers or attachments, and these are used at your own risk. None of these options are explicitly approved or encouraged by Serial 1.

With that being said, we have seen customers who have adapted trailers to our bikes.  One customer created a simple bracket that mounted to his kickstand bolts.  Another customer fabricated a hitch which is secured to the axle nut.  I have attached images of these modifications and hope they help you see what unofficial options are available to you in adapting a trailer.  If you would like a professional opinion or thoughts on the trailer, it is best to take this to a local bike shop who will have more experience with modifications such as this.