I think I can solve the "football shaped thing" quest
It's indeed an antenna, but not "DFF".
What's inside is an LP-21 rotatable ADF loop antenna. The "teardrop" housing was quite common on larger aircraft like C-47/DC-3, B-17 and B-24.
Some P-51D's in the CBI theatre used the same antenna plus housing, but it's usually been mounted right in front of the dorsal fin fillet:
The reason why it's been mounted that way in this italian P-51 is unclear.
Inside the cockpit area the football housing is simply unnecessary.
The radio used was an AN/ARA-8 VHF homing adapter.
Beacons for this system have not been deployed to europe so the whole system (receiver + antenna) was probably just dead weight.
Best regards - Mike