Hello hello.
I have to say what you describe actually sounds more like a chorus than what I was thinking of. I was meaning something where you can transpose a signal up or down something like an octave. I can't imagine how you could do that by modulating a delay line because the modulation would be too extreme.
Sorry if I wasn't clear.
Oh apologies, thats my fault. I thought you meant for vibrato style slight pitch shifting. yes, depending on how low the milliseconds go that would be a chorus or would then turn into a vibrato.
maybe its to do with, i'm not sure i can explain it well, but if you slightly, but constantly, increase a delay on a sine wave, 1ms delay per millisecond, then a eg. 500hz wave will become a 250hz wave.
the reverse of that is also true. starting with for example a 20ms delay and decreasing it down 1ms of delay time constantly per second will increase by an octave.
(i have no evidence that this was what was used in those pedals, its just something i read about in DSP books)