Fedders thermostat installed 1978. Elec furnace and a/c unit. Three parts: cover, middle plate and wall plate. The middle plate separates from the wall plate. I cannot locate the mechanism to release the middle plate from the wall plate so I can see the wiring. Help appreciated.

Maybe some screws (3) that are kind of counter sunk ,sometimes hard to see need to be removed,,

I couldn`t find any info on this part number