The 'K7' designation refers to Bluebird's "unlimited" insurance rating at Lloyd's. This is a special rating, applied to hydroplanes competing for the world water speed record. Two of the machines driven by Donald Campbell's father, Sir Malcolm Campbell, had the same rating; they were known as Blue Bird K3 and Blue Bird K4.
Other craft to have been given the same rating include those driven by Sir Henry Segrave (K1), Hubert Scott–Paine (K2) and John Cobb (K6). The last of the line (to date) appears to have been Tony Fahey's British Pursuit (K8). Wikipedia gives no further information regarding this machine, but you can see a picture of it on the Railton Water Speed website (black & white photo on the left of the third row). Reid Railton was the designer of several land and water speed record vehicles, including Sir Malcolm Campbell's two Blue Birds.
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