The Sylvania data sheet for the 12LP4 does not give a position for the ion trap magnet, but the GE & Tung-Sol sheets do:
https://www.dropbox.com/s/507h6hokl4...2007.18.07.png
I'd really like to find the correct double magnet unit, but so far, no luck. Looks like the magnet should be approx 2" forward of the base key end....mine is at about 1-3/4" and is the best it gets. The search continues.....