Thanks agin Eric for all the info. Although I found out quite by accident that the traction unit would move while being out of the seat, from day one I was aware that the blades would stay engaged if I left the seat. We have a very proactive safety awareness program at work and the ability to get off the mower while the blades were still running really stood out as a danger especially since the 455D is really industrial equipment. I made it my personal policy to hit the PTO toggle switch before dismounting the mower. It never crossed my mind that something was wrong with this until this thread and discussing the seat safety switch. I am going to take the switch into work tomorrow and have one of our electricians check it out with a voltmeter to see if it is opening and closing with the plunger action.
I have looked at all of the wiring and cannot see where anything is jumpered to eliminate the seat switch, so I can only assume that with 5000 hours on the clock, the switch was stuck in the "sit on it" position and for some reason it is now in the "off the seat" position all the time. The voltmeter should show if it is functional and hope a new switch from Century will fix the problem as getting it into them at this time of year will be nearly impossible and I have more snow in the way that will need removed.