Hard to find.  Only a few of these left. Nest Learning Thermostat Wire Base (2nd Generation), Wiring Mount Terminal:

This mount or wire base can be used as a replacement part for your Nest 2nd generation thermostat backplate. May contain minor cosmetic blemishes.

Note: This part does not work with Nest 3rd or Thermostat E. Thermostat and other accessories not included. You will receive just the 2nd Gen wiring base as pictured; quantity of one.

STORE SKU:  NEST2NDBASE