The information comes from Sir Stanley Hooker, himself, he was the chief engineer on the supercharger and later went on to be a key figure in RR jet engine development. In his autobiography, "Not Much of an Engineer," Sir Stanley relates the following anecdote:
"In my enthusiasm, I considered that Rolls-Royce designs were the ne plus ultra, until the Ford Motor Co. in Britain was invited to manufacture the Merlin in the early days of the War. A number of Ford engineers arrived in Derby, and spent some months examining and familiarizing themselves with the drawings and manufacturing methods. One day their Chief Engineer appeared in (Merlin development head Cyril Lovesey's) office, which I was then sharing, and said, 'You know, we can't make the Merlin to these drawings.'
"I replied loftily, 'I suppose that is because the drawing tolerances are too difficult for you, and you can't achieve the accuracy.'
"'On the contrary,' he replied, 'the tolerances are far too wide for us. We make motor cars far more accurately than this. Every part on our car engines has to be interchangeable with the same part on any other engine, and hence all parts have to be made with extreme accuracy, far closer than you use. That is the only way we can achieve mass production.'
"Lovesey joined in, 'Well, what do you propose now?'
"The reply was that Ford would have to redraw all of the Merlin drawings to their own standards, and this they did. It took a year or so, but was an enormous success, because, once the great Ford factory at Manchester started production, Merlins came out like shelling peas at a rate of 400 per week. And very good engines they were too.”
Total Merlin production at Ford’s Trafford Park factory was 30,428.
Ford (UK) began work in early-1940, this is about the same time that Ford (US) originally committed to build 9,000 Merlins. Ford (US) backed out a few months later, and in June 1940 a example engine and the drawings were turned over to Packard. These drawings would have been the same drawings Ford (UK) had just insulted in front of Hooker and Lovesey. Packard would have been in the same boat as Ford (UK), in that a redrawing of the R-R prints was in order.
The Trafford Park in Manchester began delivering engines about the same time as Packard, summer of 1941, even after being bombed during its first week of operation. The Merlin was built in five places, Rolls-Royce's factories at Derby (82,000 engins), Crewe (26,000) and Glasgow (23,500), Ford's plant at Trafford Park, Manchester and Packard's plant on Grand Boulevard, Detroit (55,500).