Are you rotating it, as in like you would a column shifter? If so then it's likely the NSS/backdrive setup. If you look on the engine-side of the firewall at the column, you'll still see the arm that most would only to be considered part of a column-shifted car. On the contrary, some cars actually had what's called a backdrive rod. Its purpose was so the car could not be started while in gear. A rod was connected to the detent selector on the trans, and everything moved as a column-shift car would, but the actual shifting was done by the floor shifter.
http://www.slyagent.com/tech/bdrl.htm
If you don't ever plan on installing this piece of ingenious engineering (
), use something to hold it in the park position (tack weld, or otherwise)