got out a new 501 atty today and my super T brass top would not for the life of me fit. I was pushing extremely hard and it just wouldn't snap in place.
Ah, i reread your post and i think i misunderstood you. Sounds like it was fitting fine on another 510 atomizer and when you pulled it off it wouldn't fit on the new one? O-rings sometimes swell up when in a moist environment and i suspect that is your problem. I suggest just putting on a new oring, you can get them at Lowes or Home Depot 10 orings for 2 bucks. #47 oring for the 510/901 and #5 oring for the 801.
Hope that helps!
David