XB388
Thanks John. I checked XB388 on the ukserials and jbaugher sites:
An AS5, formerly USN 69452, delivered 10/10/53 and scrapped in 1961.
These "new" Avengers were only used from 1953 for a couple of years waiting for Gannets to be delivered. I did not know there had been one at Farnborough. As you say, they were black overall, and judging from photos I have seen the roundel was further forward on the fuselage than in "my" photos. Also the "Royal Navy" logo was just above the serial on the fuselage, whereas in my photos the leters "VY" can be seen on the fin extension along the top of the fuselage. Compare the photos on p196 and p206 of British Naval Aircraft by Owen Thetford (1958).
Maybe you know someone else who knew of the Mk III in service at RAE....