Around 1972, the logo on the left side was changed to a simper one, just marked ABU (with out SVANGSTA). This is a scan of one with a foot number of 057200. The foot bottom is smooth like older ones.Then, the foot bottom started to get dotted (the serial number of this reel is 087200).
This is another 1972 reel with a dotted foot. An angular large drag wheel also started in 1972, but you can find some reels with the older small round drag wheel. I guess the leather reel case was also changed from the buckled brown one to the black double-buttoned one in 1972.
Edit (15 Nov, 2013):
I'm working to clarify when the angular large drag wheels were started to use for Ambassadeur reels. The wheel and handle (w/ retainer) of the third reel may have been changed by the previous owner.