Ok, since 24 hours have passed and this is now an open round, I'm going to ask you a question whose answer I don't know!
Here's a picture of an Italian P-51 in the IAF museum at Vigna di Valle:
A friend of mine is making a model of this very airplane and asked me today if I knew what that football shaped thing behind the pilot's seat is, and if I had any another pictures, possibly details, of it.
I'm thinking it's some kind of antenna, but, honestly, I've never seen it in any other Mustang and, despite Doctor Google, can't find any other picture or details of it.
Can any of you fine airplane connoisseurs provide the answer and the pictures I'm looking for?
The aircraft is a P-51 used by IAF post WWII, from 1948 to 1953, should be a D model, unknown batch.
EDIT: if you wanna see other (better) pictures, here's the Museum's page:
http://www.aeronautica.difesa.it/storia/museostorico/Pagine/NorthAmericanP51.aspx