[Formerly Steve Wilkinson]

Pull the speedo far enough out of the IP so that you can look at the bezel--the shiny chrome ring around it that holds the glass in place. It might have been professionally removed (to get at the odometer) and replaced, using the proper tool, typically a device that mechanically rolls the bezel smoothly back into place; an instrument shop specializing in Porsches would certainly have one.

But...if it was _not_ done professionally, somebody would have carefully pried the bezel off with a thin tool of some sort, turned back the odo and then tap-tap-tapped the bezel back into place. Works just fine--I've done it, as has anybody who has fixed a broken odometer gear--but the marks of it having been done, little ripples and tiny unevenness in the metal of the bezel on the back side of the instrument, are unmistakable.