Well the font of all things accurate (wiki) lists some 26 helicopters or autogyros used by the Germans during WWII. So it's not beyond reason that such a craft wouldn't have been available to Eastwood and Burton's opposition.
Yup ....seems that both the Russkies and the Luftwaffe operated small (Bell Sioux!) bellypoppers from about 1944 onwards.......God I remember seeing that film at the Kinema with my old man ......rattlin good yarn thobut .....Clint has a certain way with bundles of dynamit nicht wahr ? hee hee
The very best thing about 'Where eagles dare' is that Richard Burton stated that when he had made the film, his family reckoned he had arrived as an actor; he had been in a film with that proper star Clint Eastwood!