An early benefit of the agreement was to allow the UK to "Anglicise" the W28 nuclear warhead as the Red Snow warhead for the Blue Steel missile. The British designers were impressed by the W28, which was not only lighter than the British Green Grass warhead used in Yellow Sun but also remarkably more economical in its use of expensive fissile material. The Yellow Sun Mark 2 using Red Snow cost £500,000 compared with £1,200,000 for the Mark 1 with Green Grass. A 1974 CIA proliferation assessment noted: "In many cases sensitive technology in nuclear and missile fields is based on technology received from the US and could not legitimately be passed on without US permission". The UK National Audit Office noted that most of the UK Trident programme warhead development and production expenditure had been incurred in the US, which supplied special materials and "certain warhead-related components and services". There is evidence that the warhead design of the British Trident system is similar to or even based upon the US W76 warhead fitted in US Navy Trident missiles, with design and blast model data supplied to the UK.