I'll take your word for it, Retic, not having a 3.1 to look at. One question though, is the rim of the insulator (the larger end) still facing out towards the 510 threads and contact pin? The main point of having that rim is to keep the contact pin from touching inside the thread tube and base of the tank. If the rim isn't there it's liable to short out.
Yup you have complete access from either direction on the 3.1 , I just go on the side of caution as far as the threads are concerned but it can be inserted from either side of the catch cup .




