That's a Fraktur lower-case k. Here's the Fraktur alphabet. The third lowercase s the "long s", though it too looks like an f or a t to English readers; I thought it was used for words ending in s, but I'm apparently wrong. The scharfes-s (that looks like ß) stands for the ss digraph.
Indeed it's not a invention of The Nazis. They used the Frakturschrift until 1941, and then the Antiquaschrift.