I found some pics here http://www.avrovulcan.org.uk/903/index.htm

Close-up of the starboard RB199 Tornado installation on Vulcan B1 XA903. Having served as the flying test bed for the Olympus 593, a new nacelle on 903 was constructed by Marshalls of Cambridge to carry a single RB199. 593 first flew in this configuration in 1972. A Mauser cannon was at one time fitted to the nacelle and 903 became the only Vulcan ever to fire a gun.

L Braithwaite collection. © Rolls-Royce