I have seen this mentioned in another thread - but I could not find it in a search.

Found on eBay - Jazz bass control plate wired Volume, Blend, Tone with a series/parallel switch. I bought two and just installed them on my JO 4 and 5. Very nice. Seems to reduce some of the hum when soloing a pickup. Lots of tonal variation. I really enjoy the blend over two vols.

Guy who makes them, Frank, is very nice - good communication and very fast shipping.

He has a site www.amazing-bass.com

(this is a post from a happy user - not a commercial endorsement post)

I am sure these are easily made - but I am into convenience so it was easier for me to pay someone else to put them together.