I'm not familiar with the RS-1506 but service manuals typically ID the hardware used. They usually indicate the screw size and length, like M4 x 30mm. Side panel hardware can be flange washers, countersunk cup, shouldered washers, etc.
Ebay sellers may offer used OEM side panel hardware. Sometimes manufacturers use plastic washers and custom size large heads on screws you can't find in aftermarket hardware.
This is some of the side panel OEM hardware I've encountered and what I've used with custom hardwood sides. In aftermarket hardware, I try and find the largest head size available and a countersunk washer the head will just fit in. Typically, the shouldered washer ends up being an M5 used with an M4 screw. While the hole is too large for the screw, the head fits the countersink. Next to last pic compares OEM with what I could find aftermarket on some custom hardwood sides. Last pic is chart of M2 to M8 from one alloy countersunk washer manufacturer.
