Hi,
I could definately recommend the Hirsch Aligator strap (Genuine Alligator Louisiana Alligator, Large Grain USD 119.40) for the F.A Jones. It's not cheap, but it's worth it. I ordered one myslef and it amazing, just as OEM. In my opinion it's the ideal strap for any IWC, not only for the F.A. Jones, but also for the Portuguese.
http://www.watchband.biz/pi-1994967895.htm?categoryId=0
Cheers!
Greg