The RACE Electronics Sideways ROM looks like a ROM Box inside a cartridge.
From the item description on eBay:
This item allows you to combine several games / utility cartridges into one. You would remove the PROM from it's original cartridge and install it in an empty PROM socket located in this item. You would then select which PROM is to be active by poking the PROM number into a special memory location.