The contract number is mention here:
https://www.rafmuseum.org.uk/documen..._Liberator.pdf
"Ordered to contract No.W535 AC-21216 -24 from the Ford Motor Company
at its Willow Run factory in Detroit, Michigan. The order was for 4,100 aircraft
of which 3,183 were completed (1,250 of them as the 'L' model) before
production ceased in August 1945 following the Japanese surrender. "
Here is a Solar ad.
http://legendsintheirowntime.com/LiT..._ad_p024_W.png