You are correct Don, that this "touching the mountain" stamp is a printing shift.
The designers did not leave much room for error in the registration of the ink that is used for both the plane and the text labelling the stamp across the bottom. The one on the right has a shift downward from the intended position. If it is down enough that the plane's wheel touches the mountain, it becomes PA68b .
And my example of the regular stamp on the left has a slight upward shift. It doesn't matter so much with with the plane moving up a bit, but the text of "France" is now touching the design. It should be just clear of the design, with white showing between "France" and the design.
The "touching the mountain" is a little difficult to find, but used and MNH examples show up occasionally on Delcampe that will cost several times what a regular MNH runs. That's where I got my example which I found fun enough to work into my AlbumEasy airmail album. There appear to be three up on Delcampe now - one is listed very high at about 17x regular, one about 10x the regular, and there is an inexpensive used one.
France has celebrated the Concorde several times on airmail stamps as well as on regular issues. Following are the airmails:
I added today the latest iteration which was issued March 2019 to mark the 50th anniversary of the supersonic passenger plane's first flight. C82 (Yvert PA83) The design is by James Prunier. Perf 13 and a generous 52 x 33 mm sized stamp. I'm afraid my scans don't capture how the Concorde is in a slight but tangible relief and glossy so it stands out from the general background of the stamp. It's a striking effect which makes it a rather beautiful contemporary stamp.
That first flight (a subsonic test lasting some 27 minutes) was on 2 March 1969 in Toulouse France. Claude Durrens had already designed and engraved a stamp depicting the Concorde, but issue had been delayed for a rate change when somewhat unexpectedly the flight occurred. Durrens quickly added "premier vol" and the stamp was rushed out the door 7 March 1969. Sc. C42/Yvert PA43
In September 1970 the Concorde was again shown on C43/PA44 which is a bit of a mash-up commemorating the aviation achievements of Jean Mermoz and Saint-Exupery, neither of whom had anything to do with the Concorde. But it looks good climbing across the middle of the stamp.
It was also shown on a January 1976 airmail issue (Sc. C48 / Yvert PA49) celebrating the start of active commercial passenger service. The initial flight was from Paris to Rio de Janiero by way of Dakar, Senegal, traveling at twice the speed of sound, making it about three times faster than conventional passenger jets. This last one I find rather uninspiring, it certainly pales when compared with the 2019 issue at the top.
On March 8, 1979 Liechtenstein issued these two aviation-themed stamps (Sc #s 663-4) as its EUROPA set. The 40c stamp commemorates the first postal flight between St. Gallen and Schaan (1930), and the 80c value remembers the first Zeppelin special flight over the country (1931).