ACU Romchanger
From CPCWiki - THE Amstrad CPC encyclopedia!
The ACU Romchanged is a CPC6128 ROM (BIOS v3, BASIC v1.1) upgrade for the CPC464, released as DIY in Amstrad Computer User Mar 1989, pages 38, 39, 41, 42.
Very similar to the FO-DOS Cartridge.