From April 25th the first modified Lancasters began to arrive. They were all MkIIIs with the designation 'TYPE 464 Provisioning'. The title was derived from the Vickers Project Number, as the weapon and associated equipment was designed by them under the title Project 464 and' Provisioning ' due to the fact that the modification was temporary allowing easy re-conversion to standard at a later date. Gone was the mid-upper turret, and the trademark long bomb bay doors, the bomb bay now having a shaped fairing front and rear. Two heavy cast aluminium ‘V’ shaped struts were attached to the fuselage below the wings, hinged at the top against spring tension to allow them to move outwards. At the lower end of the arms was a large disc, with the starboard disc attached to a small hydraulic motor by means of a drive belt. The motor was powered by the planes, now redundant, bomb bay door system. There were several other minor modifications, including reworked hydraulic and oil systems, engine improvements and the removal of leading edge de-icing and cable cutting gear. No two planes were identical, as some modifications were found to be less effective than others, and these were modified or removed from later aircraft. The first Type 464 Provisioning to arrive at Scampton was ED909, which was allocated to Mick Martin with the code AJ – P. After initial inspection it was ready for him to fly on a 25 minute air test on the 27th April. Finding that all was well with the new bomber, he flew it again the same day on a 50 minute ‘tactical practice’. With the correct height for dropping the weapon sorted, Wallis turned his attention to the best distance. This he calculated to be 400 – 450 yards from the target, as both the Mohne and Eder dams had a pair of towers built symmetrically from their centres, simple trigonometry resulted in the ‘sixpenny’ bomb sight. A simple wooden ‘V’ shape with an eyepiece and a handgrip at the apex and two nails at the other ends. When the bomb aimer looked through the eyepiece and lined up the nails with the towers that was 450 yards. Throughout April ‘Doc’ Watson, the squadron Armaments Officer, had been de-tached to work alongside Wallis to gain experience in the use and handling of the weapon. In his absence, tarpaulin shrouded trailers began to arrive at Scampton in the dead of night. They went straight to the bomb dump, under 24 hour armed guard. No one was to go near them until ‘Doc’ returned on May 1st, by which time a total of 56 had been delivered. Uncovered, ‘Doc’ moved among the mines marking each one with a series of letters and numbers. This was his code for identifying live and practice units, only he and Wallis knew which were which. Armament crews practiced moving and loading the mines frequently, never knowing if it was a practice or if this time it was the real thing. The secrecy was intended to keep the crews on their toes, but also to conceal the fact that delivery of live ‘Upkeeps’ had been delayed….. ALL the mines at Scampton were full of concrete !. As a possible date for the operation approached, there had still been no live test of ‘Upkeep’ and no practice drops made by 617 Squadron. So on May 11th Gibson in ED932 / AJ – G, Hopgood in ED925 / AJ – M and Martin in ED909 / AJ – P flew to Reculver with concrete filled practice mines . They dropped these and Gibson reported his had a ‘ good run of almost 600 yards'.Other crews flew the following day, but as Maudsley in ED933 / AJ - X,dropped his mine he was too low and the rear of his plane was engulfed in shingle and spray as the weapon hit the water. Limping back to Scampton, the aircraft was declared Category AC ‘very badly damaged’, with no time to return it to Avro, it was handed to Flt Sgt Samsom and his Repair and Maintenance Wing, for ' their best effort '………