A603, Memory Expansion for Amiga 600
This expansion offers 1 MB of additional ChipMem, a battery-backed real-time clock and a clockport for connection of A1200 expansions. In addition to that, the A603 offers a socket for installing Indivision ECS.
For operation of the real-time clock, a coin cell (type CR2032) is required.
Source http://www.vesalia.de/e_a603.htm