According to Scott, it's the same as a sega/dataeast/stern pop bumper. This image is from his Baywatch manual. (Source https://pinside.com/pinball/forum/topic/total-nuclear-annihilation-cluball-welcome/page/80#post-4713597)
Bryan Kelly in that same thread says that Spooky added additional washers to the rod of the ring, where it would normally just be one. Not sure if that's true, your mileage may vary.
(Source https://pinside.com/pinball/forum/topic/total-nuclear-annihilation-cluball-welcome/page/80#post-4713618)
Good luck!
pop_bumper_assembly (resized).png