code: public final void setSignerRegionOverride(String signerRegionOverride) {
          checkMutability();
          Signer signer = computeSignerByURI(endpoint, signerRegionOverride, true);
          synchronized(this)  {
              this.signerRegionOverride = signerRegionOverride;
              this.signerProvider = createSignerProvider(signer);
              this.signingRegion = signerRegionOverride;
          }
      }
docs: An internal method used to explicitly override the internal signer region
  computed by the default implementation. This method is not expected to be
  normally called except for AWS internal development purposes.