Replacement RAM chip, as used in the last model Commodore 64, the Amstrad Plus range and the last Sinclair Spectrums. The C64E has two of these chips, the Spectrum +2A, +2B and +3 use 4 of them. The Amstrad Plus has two (in the 464 and GX4000) or four (in the 6128).
These chips can also be used to upgrade a Commodore 16 to 64KB RAM.