OT: Pre-war (just) Salzburg. Innsbruck & Brenner in the movies

I recently found "Above Suspicion" on iTunes. Although released in 1943, it is set in 1938 in "southern Germany" (mostly occupied Austria). Fred MacMurray is an Oxford don (and former Rhodes Scholar), and Joan Crawford is his new wife. They are recruited by British Secret Service to bring back plans for a secret weapon during their honeymoon. In an interesting casting twist, Conrad Veidt is the "good German" and Basil Rathbone plays the evil Count and Gestapo officer (who is also a former Rhodes Scholar).

There are interesting shots of Salzburg street scenes, guesthouses and the concerthall, of Innsbruck and other parts of Tirol, and a last scene at the Brenner Pass. Some of these look incredibly realistic, but I can't imagine how they were able to film on location. Possibly it is all an MGM back lot (???).