I have one and have also seen them on old ham equipment. Rickenbacker probably bought them from a supplier who also sold them to other companies for other uses. I still have mine somewhere, it came on a bass I bought too. I believe it's plausible they were original.
Mike, leave those knobs where they are...my December '73 4001 has them, too.
I called Rickenbacker about them in 1991; I spoke to a very pleasant and helpful young lady (I don't remember her name) who said something to the effect of at the time, the factory ran out of the lettered knobs, and so had to use the non-lettered ones from their amplifiers.
Thanks, Alex.
They just don't look wrong, to me, if you know what I mean...they look as if they'd been there for the life of the bass. I have a hunch they are original, but even if they're not, they could be...and by golly, on a dark stage I couldn't read them even if they were lettered...and they turn both ways, as they should, too.
All of these great posts demonstrate how with some details of Rickenbacker instruments you can say with confidence that 'there are no absolutes'. The knobs could very well be factory original, and I agree in that there were times when small parts had variations from what most people see from time to time. These knobs are a pretty good example of that.