bomb aimer had the task of operating the H2S during these periods. To further improve operations, on 12 March it was decided that Bomber Command would receive more of the available spares, as it was believed that they would need to make up for higher casualty rates. Previously, every equipped squadron was required to hold 100% spares for all parts, and there simply were not enough to go around. H2S Mk. II, production version The original H2S sets were essentially prototype units that were hand-built to equip the Pathfinders with all possible speed. Among the many problems with the rushed service entry was that the developers were forced to use existing plug-and-socket designs to connect the various units of the complete set together. There were no bulkhead mounting male connectors available at this time, and consequently many of the male free connectors at the ends of cable runs carried exposed lethal voltages. While installations of the prototypes progressed, work was underway on a true production version, the Mk. II, which would go on to be the most numerous version built. This was largely identical to the Mk. I's with the exception of various packaging and electronics details intended to make them easier to build. Bomber Command started general use of H2S in summer 1943. On the night of 24 July, the RAF began Operation Gomorrah, a large attack on Hamburg. By that time, H2S had been fitted to Lancasters, which became a backbone of Bomber Command. With the target marked by Pathfinders using H2S, RAF bombers hit the city with high explosive and incendiary bombs. They returned on 25 and 27 July, with the USAAF performing two daylight attacks in between the three RAF raids. Large parts of the city were burned to the ground by a firestorm. About 45,000 people, mostly civilians, were killed. The Mk. II was soon upgraded to the Mk. IIA version, which differed from the Mk. II only in the detail of the scanner antenna; IIA replaced the original dipole antenna at the scanner's focal point with a feed horn that sent the signal back to the receiver in a waveguide, eliminating the lossy coaxial cable of the earlier model. Scanning improvements It was noted on even the earliest flights of V9977 that a number of basic features of the H2S made it difficult to use. Attempts to fix these began even before H2S entered