I don't think it's a bomb. I remember having seen something like that - If I am not mistaken it's the warhead of a heavy ground to ground rocket. But I may be wrong.
This is an inaccurate Allied reconstruction of the PC 1000 RS. They captured a bomb with its light alloy tail broken to pieces an reconstructed it wrongly - as too long, with just four fins and mounted backwards. They were also fooled by the warhead size and identified the bomb as the 500 kg class.I have forgotten