The Bell Labs A-3 scrambler used real time band inversion and transposition and was 'snake oiled' into the commercial market, but under the pressure of WWII it fell quickly. It was bad enough it was self-clocked and a German engineer had helped design it. But even more embarrassing was, without having to reverse engineer the circuit, humans could train their ears to recognize individual speakers and even words.
Today we take for granted the ability to conjure a complicated pseudorandom digital stream for keying, but in those days it was just "no can do".
In WWII... SIGSALY was the first system secure by modern standards. Pairs of synchronized one-time phonographic records containing a sequence of tones seeded from a noise source.