The US provided GB with some P-38 Lightings (before the US involvement in the War).
These came with the US built Allison engines, but, without the "supper chargers".
The Brits called them "Castrated" Lightings.
The Merlin engines really help performance in the P-38's.
But once the Brits starts supplying the Merlin and its specs to us to build, they got the "Supper Chargers" specs.
A win win situation for everyone, except those who had to fight aircraft with those engines in them.
My best,
Blair