The 50 number is incorrect like most numbers on the internet, in the press release by Microsoft they mentioned shipping a 1000 of these out to developers. The hard part was keeping them after being received as Microsoft wanted them shipped back before getting the actual development kits.
Nice find, its also a standard ATX case so you could build a current PC. It's a shame it didn't stay in the box it shipped in as so many of these cases are beat up. This one is in good shape compared to others I've seen. The shipping label on my boxed unit has the date 4-19-2001 and the new case inside was dated the same as yours, 4-18-01. Which is a late date for an Alpha tower and probably why it wasn't assembled as the BETA 1C units were currently shipping.
Board Life Status
Board startup date: April 23, 2017 12:45:48