If you look at all the direct-from-China vendors that sell this cap:
https://www.ebay.com/itm/132000300755
https://www.ebay.com/itm/132315518596
It's obvious that it comes with 2 O-rings for proper fit. Not sure why SSR one doesn't. Probably to save $0.50 and make bigger profits.
As for dymmy holes, there's only 3 threaded holes on tank at 1, 5, 9 o'clock positions. Cap would look wierd with just bolts at those locations, so 3 other holes are included with dummy bolts to make it look more even with 6-bolts every 60-degrees.