Am I tripping or is the linked manual for the Mark II, not the Mark I?
So both the handwritten title about Ferranti Mark I and the typed title about Manchester Mark II are right. Only the name of the PDF file is wrong.
Manchester Mark I had been an earlier and simpler experimental computer, while Ferranti Mark I was a commercial product. It actually was the first commercial electronic computer, with its first delivery a little before UNIVAC I.