I got a M1A1 with both size tanks. Compared to other tanks it's kind of strange because both caps are so easy to remove & insert....but they sit so tight on the carto & never slip. Big plus in my book. I've bent my thumb nails back so many times trying to remove a few other tank caps. I can take my provari w/extended cap & 18650 & shake the hell out of it & it stays put. I have no worry's about keeping it in my pocket & do it at work all the time. Also you don't have to worry about the tank cracking or splintered on it. It has a lifetime guarantee.
Also, they now make a single coil boge that fits the DC tanks & I use them in my DC tanks. Just find the one's that have a tube length of 45mm, over all 50mm.
As far as the screw, don't worry about it because it actually has a few benefits. I used to use a syringe to fill, which I HATE, & the filler hole made it so much easier to fill. I ended up getting one of those plastic drip tip filler things from clouds of vapor ($4) & now happilly retired the syringe, except to remove juice to clean & change juice. Anyway..I've found when I'm drinkin'..I mean chain vapin, that if the carto starts getting a little dry, I can remove the screw & immediately reinsert & it seems to funnel better. You can't do that with another tank.
Donna's found some really nice screws & given good descriptions if you decide you don't like the plastic they come with it. They really are pretty sharp imo.
One more thing & I'll end my ramble...when I take the M1A1 tank & rinse under hot water, the smell of the previous juice disappears faster then other tanks. Which tells me it's less porous.