Hi Wilco, welcome to another Dutch member!
I wonder if you have a custom Brown Bass with a historically correct fixed brass nut - the new Brown Bass comes standard with an adjustable nut.
The adjustable nut has three big holes between the string grooves, for the allen bolts (inbus-bouten). The two on the outside go up and down to set the height, the middle one goes up to unlock the nut and down to lock it again (or was it the other way around - I keep forgetting).
There would also be a 1 to 2 mm thick base plate in brass that the allen bolts work against.
People will start banging bottles of beer on the table soon, demanding pictures - and before you know it the thread starts discussing motor bikes.
(Message edited by adriaan on February 27, 2008)