Johnny's switchblade was real. It'a a German "Springer" lever lock "shell-puller". It is one of THE most discussed, debated, and studied switchblades among vintage switchblade enthusiasts (and fans of the movie). The model/type is also one of the most sought after by collectors specifically because of their fondness for the movie.
Although I don't share such interests, I did do my best at one time to research the knife. Below is the best close-up screen shot I could ever get of the knife showing the tang stamp, which likely reads "Made In Germany". If it was a prop, I don't believe the maker would have bothered to add a tang stamp, particularly since the only way you can see it is if you freeze the frame at just the right second, zoom in, and have high-def. Also, since many such knives have been made, by a variety of German knife makers, it really wouldn't make sense for the prop guys to make one from scratch. They are rather complicated knives.
Above the tang stamp, on the ground portion of the blade, there is something etched or engraved, letters?, numbers?, a detail that is also much discussed and debated. No one knows exactly what it is since it's not common on German springer switchblades.
There are also many guesses regarding who the German maker is, but I have never seen definitive proof, nor have I ever found one with an identical tang stamp.
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