SH-4 is the JB model, isn't it? See:
http://www.thomann.de/gb/seymour_duncan ... pickup.htm
I haven't seen replacement pickups sold for neck or bridge position, at least the JB. It's just the JB.. I don't think there's different neck and bridge models at all.
Btw, which pole spacing is right for Jag-Stang, Should the pickup be SH-4 or TB-4? See:
http://www.thomann.de/gb/seymour_duncan_stb4bk.htm
(Yeah I know it says "Floyd Rose" on TB page, but the SH and TB models differ also in pole spacing as I've heard it)