Sensor of Roland.
Price usd6 to usd8 per piece is for generic sensor,not original roland sensor,but work totally fine.
Those sensor in this price inluding te following model:
Roland AJ-1000/XC-540/XJ540 SENSOR-INTERRUPTER GP2A25NJ-15099115
Roland SENSOR INTERRUPER GP1A05A5 -15229506
ROLAND RA-640/VS-640 SENSOR INTERRUPTER,EE-SX4009-P1-1000006689