EFI firmware for Apple logic board 820-3115, revision B, marked UNLOCK ok, meaning the firmware password lock has been removed and the image reported working. It is 8 MB, matching a 64 Mbit SPI ROM, and has no file extension in its name. Apple's 820 numbering identifies the logic board itself, so confirm the number and revision on the PCB silkscreen. Flash with an SPI programmer after reading and saving the original ROM, and restore the unit serial and system data from that backup if needed.
The button opens the matching file on RM Schematics, or a source page if it is not catalogued there yet. The original attachment is from @lappysan. Enginerys does not host this file.
Back up your own chip before writing any dump, and match the full board number and revision. Flashing guide →