Hi Erik,
Only one of your photo links seems to work.

Yes, this is a 3646M Autavia, it's a relatively early Autavia. Year would be '68/9. They aren't rare, but they are thin on the ground.
Parts for these aren't readily available, you could try posting a WTB notice in the trader forum (link is upper right).

Its referred to as 3rd execution dial; 3rd execution hands (matchstick, with luminous inserts). These cam with either a Minute bezel insert (M), Hour (H) or Tachymeter (T).

Pics here: http://www.onthedash.com/Guide/_Chronographs/62.Autavia/a.Manual--Screw-Back_Case/2nd_Execution_Screw-Back_Case/3646_Second_Case_Third_Dial/

And further details in this table - row 'B-4'
http://www.onthedash.com/Guide/Master_Reference.php4

Stewart