I've always thought it was a very slick design. The view in the ad stresses the smoothness the chamfer/bevel transition from one case to the other. Very nice indeed.

For some reason I'd always assumed it ended short of the power plug and switch. It took me by surprise again when I re-discovered this pic. It looks to me like it probably has a built in power plug and the switch cuts power off there, with the Compact's switch in the on position underneath. Dollars to doughnuts, there's a shorter version of the passthru SCSI port as on the Clipper Drive under there.

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I wonder if they had a different model for the SE/SE/30 cooling fan setup?

The somewhat less elegant approach I've come up with would be bolting a modem case up to the backside of the Compact. Not quite Kosher, but that's the kind of utilitarian hack I would have done back in the day. It's silly to have the modem a large modem taking up desk space. I think I've got an Apple(?) modem around here somewhere I've been saving for that 10" CRT as an option if I decide to put it into a real compact instead of a ProtoHoaxMac.