Huh interesting - if you go to pinside classified archives / Gilligans and look thru the pics (click to enlarge), they all seem to have a white plastic plug like you noted. These show it pretty clear
https://pinside.com/pinball/market/classifieds/archive/101748 (15/24)
https://pinside.com/pinball/market/classifieds/archive/127357 (23/31)
https://pinside.com/pinball/market/classifieds/archive/120619 (3/11)