Spotted on eBay, a transparent Newton (MessagePad 110) with its accessories. The price is high ($1,100), but not outrageous.
Well, as it was pointed out to me, this is a limited edition (100 units?) intended for developers, not a prototype. Still quite rare, though.
In addition to the Newton, there’s a Mac connection kit, a charging station (still in its blister pack), two keyboards (one still in its blister pack), 1MB, 2MB, and 4MB memory cards, a power adapter, and batteries.