Here-
http://forums.vintageguitars.org.uk/showthread.php?t=2051
Never saw one before.
I'm getting that the link is no good. I followed it back to the site & I think they moved it (because it's not a Rick & he posted in Ricks is my guess???)
If it's the one in "Other names" I'm afraid he'll be disappointed. I saw two just like it on Ebay in the past two years, kept a pic of one of them somewhere. John was a bit of a rascal & would tell you what he thought you wanted to hear, he was also very old & had a bit of a foggy memory sometimes.
He died in Oct/Nov of 2000, just after building my bass, the ones online now are made by his buisness partner who's been OK'd to use John's name. £1400 your cash!
Magnum pups are incredible, at the time 70's/80s they were by far the loudest available, hence their use in Metal/rock guitars & basses. I've got two in my Birch bass & one in an SG Junior. It's impossible to get a clean tone out of the SG but it does cut oak trees down at 500 yards