Read through here and it sounds like you have gotten some solid advice. Speedo cable has 2 O rings..simple fix. Your initial pic is of the pan and not rear main seal. Further along Inertia posted a pick of the rear main seal that is rubber. With those tabs I an not sure how you would replace without motor out of car....you can's slide that sucker. !
Either way, just taking the rear main cap off won't do anything to get to the upper, because it's above the crank, in the block, itself, along with the rubber blocks that seal the main cap to the block and the lower, within the main cap..