There are some F bodies that had modifications done by third parties to provide the mirror prefire feature. The most well-known modification was done by the Questar telescope company. Added a second button that looked like the self-timer button. You'd depress that, then rotate the mirror lockup switch to lock the mirror up.

Supposedly, early F bodies can do the same if you depress the shutter release part of the way down. Later bodies didn't have feature from what I've heard and just verified with my F. An early F is soon to be on its way to me. Will check that camera to see if it'll do it or not.

-J