The AFMER is for 07/10 FFL Manufacturers to submit annually, for identification of firearms manufactured and distributed into U.S. commerce or exported out of the U.S.
If an FFL Manufacturer has transferred a pistol equipped with a stabilizing brace to another FFL Manufacturer, and that second Manufacturer now registers it on an ATF Form 2 (as an SBR, per the Final Rule), the second Manufacturer distributing the registered SBR into commerce must complete the AFMER for that weapon.
If an FFL (dealer/pawnbroker/importer) has within their inventory a pistol equipped with a stabilizing brace and opts to submit application to register the SBR (on an eForm 1), they are not required to submit an AFMER because the firearms already existed in commerce.