From Ascalon Arms:
Receiver shim to delete the clearance and play between upper and lower receiver.
For reasons of compatibility and easy assembly, the factory CZ Scorpion EVO 3 has greater tolerance in the area of the mounting pin. This causes a certain play between the upper and lower receiver (see video). This receiver shim has no effect on the interchangeability of parts but removes the unpleasant movement between both upper and lower receiver.
It is designed to work with the original polymer handguard on EVO 3 S1 Pistol or Carbine which both have a flat bottom of the upper receiver/handguard at the contact point. There might be a compatibility issue with some other variants or combinations of aftermarket parts.
Some very early pieces might be missing the rib in the lower receiver which the shim uses as an attachment point. Also the hole where the shim is supposed to fit is smaller. In this case the shim won't fit in the receiver as it was designed to do.
Receiver shim is compatible with CZ Scorpion EVO 3+ as well.