In all the accounts I've read of Hitler, by William L. Shirer and John Toland as well as others Hitler was only ever decorated with the Iron Cross/Second class which would have been a rare honor for an enlisted man.
In accordance with German military regulations the Iron Cross Second class was worn over the left breast pocket and could be worn with or without the Second Class ribbon which would have been looped through the second button hole on the tunic.
The Iron Cross First Class was always worn at the collar(or throat).
I've never seen a picture of Hitler wearing an Iron Cross First Class.
Good observation. It may have been historically awarded after he rose to power.