Internal 512 KB RAM expansion for CPC6128 from german magazine c't, issue 10/1987. The DIY project name is CPC 6512.
One half (256K) of the total (512K) expansion memory can be accessed as Video RAM (with normal expansions, only 64 KB are accessible as Video RAM). So, the c't expansion might be incompatible with Standard Memory Expansions, or it might be a backwards-compatible variant with additional features.
Technical
I/O Ports are unknown ...?
Scanned Article / Schematics
c't, issue 10/1987
Offtopic
The CPC 6512 c't Memory Expansion is not to be confused with issues CPC 6512 and CPC 6513 of the more popular World War II Fanzine from Concord Publications Company.
CPC 6512
CPC 6513
