Hi Lasse,
I don't know my way around the Elan control cavity well enough to be confident, but I did notice a couple of things:
1. As you pointed out in your serial number request thread, there doesn't appear to be any change to the location of holes in the top. However many were there to begin with (at least volume, pan and filter?), they seem to have been in a straight line except for the Q switch.
2. Just a hunch here, but I believe five knobs would have been the magic number for an East-Meets-West package (volume, pan, filter, bass, treble). That might be where where your bass started out, and might account for some of the components in the control cavity.
When the serial number lookup comes back, you might find out that the only cutting a drilling was for the pickups. It might also be possible to order a complete electronics package (or whichever parts were removed during the modifications) including new pickups and controls and have your bass back to the sonic factory specs.
I hope this is helpful.