Mountain Computer Expansion Chassis.
If you own many peripherals for Your Apple II computer, you have probably run into the problem of too few slots for too many peripherals. The frequent “card swapping” you must do to cope with this problem is inconvenient and can be damaging to your peripheral cards. The Expansion Chassis offers a solution to this problem by providing you with an abundance of peripheral slots.
The Expansion Chassis is compatible with Pascal, as well as Integer Basic, Applesoft Basic and the Monitor. It can be controlled with software, hardware or a combination. In addition, each Expansion Chassis has its own power supply and provides 30 watts of continuous power (info from original manual from Mountain Computer Corp)