Well, it's obvious because the spring in a 504 is smaller than the double spring in a 5108 as the 5108 head is much heavier... so the 108 head doesn't run up and down smoothly now. Well, it runs down smoothly. So they need to be swapped. I'm just trying to find out the right way before I get in a muddle doing it the wrong way.
I'm in England, near London, and have asked on FADU as well, and also emailed odyssey to see if they can help.
I can certainly put up some snaps. I'll try over the weekend.