iMac 21.5-Inch Mid 2014
To open the Mac you cut the tape holding the screen down as Apple decided to glue the screen in...
The ROM is a SOIC-8 chip (64Mbit) that can be read or written by the Medusa-3 with a Pomona clip.
The ROM in on the top (easy to reach) side of the motherboard.
The board number is on the left edge of the board, about half way up.