I've been trying to find that out for months. The closest answer I got was from Evan Skopp, saying that it was specially wound for Walsh by his buddy Seymour. It clearly has the SM-series cover on it. That cut with James Taylor is one of the best guitar tones I've ever heard, certainly the best Strat bridge tone. I emailed Walsh through his website, but no reply so far.
Let me know if you find anything else out about that pickup.