I never had one of these, but I did see them when I visited the commodore club at my home town.
Later I wanted to have one and found one for sale on the internet.
I’m still looking for another one as the one I have is missing it’s keyboard cable and have some holes in the cover as the previous owner used it to control a model train and had mounted a thermoplastic connector on top of the machine.
The SX-64 is almost 100% compatible with the Commodore 64, but it is missing the datasette port and the userport has 2 pins shorted.
The SX-64 also have it’s own version of the kernal rom, it has changed startup collours and is missing all tape routines.
Early systems also had problems with cartridges as the flatcable to the connector was not correct shielded.
The SX-64 never failed me, and is running good.
The inside: