I've wondered that myself - the 510 cartos manufactured by SLB have that internal ring (donut shape on top of the stuffing, under the end cap). I fill the normal way (remove the soft-cap, drip into the stuffing) and that ring is a pain - getting it out isn't hard but you have to be careful not to squish the stuffing putting it back in.
Are your end-caps soft? If not and you mangle the end cap taking it out, you could probably use one of the LiteCigUSA.net plastic 510/901/KR808D-1 plastic drip tips as a mouthpiece on the carto. I use them on KR808D-1 cartos.
And BTW you can get a D2 to D1 adapter at leCig.com and then use all of the Vapor4Life cartos (soft-cap, no internal ring) or any of the various KR808D-1 standard-resistance blanks (not LR low resistance - those are for fat-battery models with beefier electronics and can damage slim
batteries).