I think I finally found a mechanical replacement for my Microsoft Natural Ergonomic Keyboard 4000 and Surface Ergonomic Keyboard. The Perixx PERIBOARD-835 : r/ErgoMechKeyboards
Natural Ergo Keyboard 4000 by Microsoft : ErgoCanada - Detailed Specification Page
Amazon.com: Microsoft Wired Natural Ergonomic Keyboard 4000, Retail : Electronics
Microsoft 4000 V1.0 KU-0462 1048 lifter Natural Ergonomic Keyboard USB Stand A | eBay
Ergonomic Keyboard 4000 Review of the Microsoft Veteran
Amazon.com: Microsoft Wired Natural Ergonomic Keyboard 4000, Retail : Electronics
Ergonomic Keyboard 4000 Review of the Microsoft Veteran
Amazon.com: Microsoft Wired Natural Ergonomic Keyboard 4000, Retail : Electronics
Buy Microsoft Surface Ergonomic Keyboard 4000 - Microsoft Store