I opened my door this evening to find that UPS had dropped off a large cardboard box; inside was a nicely configured PowerMac G4 450Mhz DP system. There’s no note or anything in the box, just the system unit and lots of packing peanuts.
Update: Since originally posting this, the mystery has been solved. Thanks to B.D. for the PowerMac G4!
This now makes two machines that people have donated in order to help me support people still running OSX 10.1 through 10.4. J.G. in Sunnyvale sent me a “Blue & White” G3 system, and the UPS label indicates that the G4 came from Chicago.