I do not think that Key pad will work on a GRE PSR-500. The 106/651/1040 configuration is different - Speaker on bottom of face plate and Sel button more square. PSR-500 has speaker on top and SEL button more oval.
Over time the material glued on the bottom of each key which makes contact with the circuit board gets old and flakes off. These flakes can cause a short (button to get stuck) on the circuit board. If you open the scanner and take out the keypad you can blow out the flakes that are shorting the circuit board. Most likely it will continue working many years afterward without further problems. If a button on the keypad stops working altogether there are people that sell keypad repair glue on e-Bay at times. You simply apply some to the keypad buttons to repair the contacts which have worn out and flaked off.