That negative is definitely flipped. Look at where the "beehive" feedthrough insulator for the fixed-length wire antenna is located. It's nearly at the aft end of the radio compartment, which is right where the insulator for the fixed-length SCR-287 antenna would be. The insulator for the SCR-274N command sets is near the front of the compartment, because that's where the command sets are located. I've seen photos of some Forts with TWO fixed-length antennas running from the sides of the fuselage up to the vertical fin. I'm betting this is one of them.
Cheers!