Some of the German states used the helmets post-war and these are seen with the flags on the side. Also, the transitional WWI helmets used in WWII can be seen with decals. So it possible that either of these could be what you've seen.
But I think even the Free Korp helmets are rare. Those with big swastikas on the front or skulls and cross bones. Who knows, those could have been painted that way in 1950 and given the aging you'd never know when it was done today!