I remember seeing a documentary on this device and how it took some time to implement a code change as the ROM which was core memory had to be re-knitted. Either the sense wire went through the core or over the core for a '0' or '1'. Ah! the days of REAL computers.

For a real freak out, find out how they used cathode ray tubes and mercury baths as memory. A search via google turns up some interesting artifacts.