The bonds show every sign of being genuine! It is illegal in Italy to import or export more than 10,000 euros worth of financial instruments including currency without prior permission. This is an anti-money laundering measure.
If the story is true - and it's not currently being reported by the major news organisations so some scepticism is called for - then two possibilities exist:
1/ This bonds are real. In which case they are most likely not stolen since that would have been major news. So were the two Japanese gentlemen acting on behalf of the Japanese government - lawful holder of a vast amount of Uncle Sam's debt? Perhaps Japan is "exiting the dollar" covertly.
2/ The bonds are fake. Some of them are apparently bearer bonds dating back to the Kennedy era and might actually be redeemable if the forgery is good enough. In this case Bernie Madoff has just been deposed from his "biggest criminal ever" throne.
It will be interesting to see how this one pans out.