Maestro Parametric Filter MPF-1 Guitar Effect Pedal Late 70's - Early 80's. The back plastic battery cover has been modified to allow a 9V power supply to plug in instead of batter power. The pastic piece looks to have been broken in half length wise and glued back together. The plastic plugs that held the battery cover no longer function. 2 nuts were glued to the inside of the unit that allow the 2 bolts to hold the battery cover in place. The 9V plug could be bypassed and a battery can still be plugged in, however, that would not be advisable as regular removal and replacement of trhe battery cover could losen the glued nuts.