I always put mine in with the inside section that has the bevel Up or outside the tank so that when I installed the filler tube It would slide in easier.
My reasoning was if you removed the tank for some reason it would be easier to re-insert the filler tube, plus how many times would you remove the grommet Vs the filler tube for ohh say sending unit removal or line replacement. so my best guess is rounded inside the tank and square lip out. I ahve seen them in both ways and they did not leak either way, just a bit easier to re-insert the filler tube with the bevel side up.
