So we are talking 722.3/4 transmissions. The B2 band piston was redesigned early in the life of that transmission series. 126 cars would probably have the early band. The redesign was to the outer sealing edge. Not only does the late seal work better and cut wear in the bore, I think the valving in the piston was redesigned to aid in un applying the band (has to do with fourty gear flare).
The piston can be replaced in the car in most cases. It will be included in any competent rebuild. I don't think I would worry about it if the trans shifted good into 4th.
Bosch Master, ASE Master, L1
33 years MB technician