Here's an excerpt from the VGI website, long considered to be the authority on vintage guitars:
This pictures shows the two roll off mini-pots and switch from the bottom side of the pickguard. These roll off pots allow the player to "pre set" a tone, then turn the tone controls completely off or on with the toggle switch (one tone roll off pot for each pickup).
(This is from his "Inside a '58 Jazzmaster" article)
I'm not picking on the VGI guy, he truly is a wealth of information, and his site has inspired so many Fenderholics. And I applaud him for even having a Jazzmaster article on his site (the pics are great). But he obviously did not play that Jazzmaster he did the article on. If so, he would have been hooked... the article would be correct, and the JM would no doubt be rated higher than an C- on his scale!
Just goes to show that the JM still has not completely "grown up" in the world of enthusiasts and collecting... which is both good and bad for us. Had he played the Jazzmaster... we would all be paying even more for them then we currently are!