When Mannix is walking through the open room with rows of desks end employees and goes through enclave before going to his office, the footage is flipped. All the (rotary) phones have the red hold button on the far right, and in the enclave, the clock on the wall is backwards. During this footage Mannix is wearing his gun on his right, to draw with his left. Mannix is right handed.
When in his office, Mannix has his gun on the left as usual.
When Mannix is talking with Montoya at the observatory, close-ups show a sold gold-colored wall behind Mannix but longer shots show an open doorway behind him. Obviously, the close-ups of Mannix were shot in-studio in front of a plain wall.