I’ve had this cartridge expander for a few years and always assumed it was commonly known about and that the ROM inside had been dumped. It seems that may not be the case, so this page is to provide the information I have about it.
The “Game Port Expander 40/80 Column Converter” from MJB Computer Products LTD is a 4 port cartridge expander for the Commodore 64. It comes in a rather sturdy brown metal case. It plugs into the cartridge port and provides 4 additional cartridge slots. A dial on the right selects which slot is active, indicated with an LED. The C64 LED is for 40 column mode (no cartridge). There’s also a ROM inside that provides a soft 80 column mode. The 80 column mode doesn’t seem unique, using the same half-width characters that many others used. The color scheme is unique though with a red border, white background, and blue text.
I’ve dumped the 4k 80 column ROM to a BIN file to burn to an EPROM and a CRT file to use in emulators like VICE.
An old auction provided a glimpse into the “manual” (a single page) for which I’ve re-typeset and provided in the archive.
You can download the ROM and the manual here.