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Sunday 17 December 2017 at 07:07
Early ones and Gottliebs had the rounded corners. At some point, Williams designed the ring to slide over the new narrow bumper body which meant you no longer had to take the body off or unsolder the lamp socket to change a rid and ring. But that didn't mater for gottlieb due to their body design....
Source: http://pinside.com/pinball/forum/topic/gtb-pop-bumper-ringshafts-different#post-4127885