The SRWC is a special design quick release weaver rail clamp specifically designed for RC220 and RC500. 

It's original design purpose was to convert the PowaBeam Light Bracket to a quick Release option which can be bought with the rail in a complete kit.

https://www.powabeam.com.au/smartrest-quick-release-adapter.html 

The package includes:

• 1x Quick Release Clamp (SRWC) 
• 1x Fastener Kit