It will have a seal. The only time i saw an o-ring on a selector shaft was on a very early THM 400. I doubt that the 350's had them when they came out, and even if they did, chances are that your 350 is not the old. Beware that you need a special tool to remove the seal. I would be very difficult to try to knock it out and you would be risking the selector shaft as well. If you do get it out(if that is indeed the problem) insepect the shaft for ridges, rust, knicks, things that can damage the seal on installation.