BTB775s did not have the bridge pickups as far as I know . The bridge pickups were in BTB 785s .
Also here is a link to an Ibanez owners manual , it shows how to adjust the bridge pickups for Ibanez basses . http://www.ibanez.co.jp/world/manual/guitars/EDA.pdf
The five holes under the bridge are the individual string volume adjustments as has already been said here . With the other two holes , the one closest to the bottom of the bass is the overall volume for the bridge pickup to balance it against the volume of the magnetic pickups . The other hole a little higher up is an EQ adjustment for the brightness of the bridge pickup .