Shielded RAM expansion made by Commodore to their Amiga 500's trapdoor slot.
It have 512 kB on the board as well as a battery backed-up clock (for Workbench).
First type (up to Rev. 5) had 16 DIPs of 256Kx1, from revision 6 (1988 and onwards) it had "only" four DIPs of 564Kx4
For Amiga 500+ there came a the A501+ expansion, this had 1MB on the expansion board, using eight 256Kx4 DIPs.