For Sale: £500 CAAD5 Optimo Capo SS Bike Complete 56cm

it is not a freewheel hub.
Its a track hub

Hubs: Shimano Dura Ace Track HB 7600

do a google search on these and come back later.
it has a bmx style freewheel attached to the track hub threads:

A standard single speed BMX style freewheel can be used on the track threads the hub. Because of the stepped down lock ring threads, not as many threads of the freewheel engage the hub threads as they would if it were threaded to an freewheel threads, however, this is not a problem since the same number of threads are sufficient to secure a track cog. The force applied to a freewheel is the same as that applied to a fixed cog of the same size, so the missing threads cause no durability issue. Also, there is never any reverse torque on the threads with a freewheel, so a freewheel actually puts less strain on the threads than a fixed cog does.

from wikipedia:http://en.wikipedia.org/wiki/Flip-flop_hub

all you do is remove the freewheel, put a track cog on, put a track lockring on and its a fixed bike.