The Radio Compass DF Loop LP-21 antennae used for multi-engine US aircraft like the B-17, B-24, B-29 B-25 and many others.
The full inscription should be :
LOOP LP-21 ... <- and here one or two letter/s indicated the type. Clean slightly the area up to find the last sign/s.
NONSTATIC
CORRECTED
T.O. ...-...-... <- here the Technical Order number. Your seems to be 2-3-3? but it was overpainted. Also required cleaning at the end sign to fine the correct ending digit.
DATA LINE NO. ... <- here a number also cleaning of the area is required to get it.