[QUOTE=GrahamSimons;1785821]Some interesting comments here – many thanks!

Peter Clegg, who was in contact with Margaret Dove, Roy Chadwick’s daughter for many years, assures me that Chadwick visited the US and Canada a number of times during the war and visited Northrop ‘at least once’. During one trip there was a meeting with Dr. Norman F. Ramsey of the Manhattan Project in October 1943 for example. Ramsey seized the chance to show Chadwick some preliminary sketches of both the ‘Thin Man’ gun and the ‘Fat Man’ implosion weapon casings. Chadwick assured Ramsey that the Lancaster could accommodate either bomb and promised whatever support might be needed.

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Ah No – Ref your Fat Man claim -In Oct 1943 plutonium had not been produced in a quantity sufficient to determine its neutron cross section and likely PU240 concentration, which are vital measurements for determining how it would need to be assembled into a critical mass. These measurements were first preformed in April 1944, and showed plutonium to be unsuitable for a gun type device. The implosion concept surfaced as a response to this discovery in the late spring/summer of 1944.

Hence Chadwick could not have examined Fat Man sketches in 1943.