Far as i know Commodore only ever had this for the Vic (thought the 64 would physically fit the color would not match, you would have to remove the cartridge port expander, and the 64 had the Educator model to look like a Pet anyway so it wasn't really needed. The ones made that allowed you to fit in two 1541 drives by Xetec were more functional and they even had a 128 version that fit two 1571s), and multi cartridge adapters weren't as useful for the 64 anyway. The Vic, since it came with less RAM than the processor could access, could use more than one cartridge at once and this was how you would get to the max RAM for the Vic, by having a few of those RAM expanders on a cartridge port expander. A similar expander for the 64 requires access to turn on and off cartridges you don't need to use the one you do need. The 1020 I have is missing the cartridge expander which is a pity since the one it comes with, from pics I've seen, actually has a port out the back to place cartridges on and off without having to open the case. It does make the Vic look very Pet-alike so I bought it anyway and use another expander in there, or my Behr Bonz cartridge which eliminated the need to swap game carts anyway.
Also of note, the person does not have the cassette hooked up and in the case, they are just using it for storage and there are holes to run the cables out of the case to have disk, printers, cassette outside as well as to hook up the monitor. Note, the RF adapter that would hook to the top of the case near the back (the rectangular box with the chicken head logo). That connected to the Vic display port and had a hole for its RCA port right through the case cover there.