Old versions were TX, TXP, and TXT. TX was a nominally 400 speed film that came in 35mm and 120 (and other rollfilm sizes in the course of its history). TXP and TXT were nominally rated at 320 and had a longer toe, meaning they were less forgiving in terms of exposure at the shadow end, and were designated "professional" or "studio" films, though anyone with good exposure technique could certainly get good pictures with them under a variety of circumstances. TXP came in 120 and TXT came in sheet sizes.
Now there is TX 400 (in 35mm and 120) and TXP 320 (in 120 and sheet sizes), and they are supposed to have the same tonal qualities as the old versions, with greater resistance to dust, finer grain, and slightly different development times. As these new versions come out, people are testing to see whether the claims are true. I haven't tried them yet (still have about 50 rolls of TX 120, and 100 sheets of TXT 4x5, and about 40 sheets of 8x10", and 6 of 11x14" before I have to try the new stuff--I'll let everyone else work out the kinks first).
