Greetings!
Problem solved! Tech support had me remove the "insulation disc" from the underside of one of the solid plates and attach the TH sensor there (the sensor hanging down), which is now the top plate. I just used one of the disc screw holes to mount the sensor board.
Then are now the 2 "open" plates
below that top plate, then two solid plates...the bottom plate being the one with the threaded screw holes. Davis never mentioned the 6th plate solution that
xairbusdriver mentions, which I noticed in some photos.
Oddest of all though is that before I disassembled the shield, the five plates all fit together (including the one with the insulating disc...whatever that does), with the sensor board face up on the one-inch standoff screws.
FYI the TH sensor board was $73 USD from WeatherBuffs via Amazon.
https://www.amazon.com/s?marketplaceID=ATVPDKIKX0DER&me=A16021BDEPIY71&merchant=A16021BDEPIY71Finally, in spite of the paucity of online documentation for the shield, the Davis tech service guy responded quickly and helpfully throughout this adventure.