If there are no exposed polls, and they are sealed in...I dunno, it feels like silicon. The manufacturer says that they are rails so they can be used with either 4 or 5 string basses, but with some past experience, I've come to learn that they might be completely making that up. I though if I could get some scrap iron or steel and grind it down and collect the dust and put that on a sheet of wax paper maybe if I put the PU underneath it would show the pattern..is there an easier way? And, does it really matter that much if it's a wide 4-string and I use it for a 5-string bass? I tried a 4-string electric mandolin that someone build (not incredibley well, either) with a 6-string guitar pickup, and I really couldn't notice any volume drop betweenthe strings...will the same hold true for a bass?