this was a comment I posted on rec. aviation.homebuilt back in 2009 before the trolls destroyed usenet.
I thought I had cracked the problem when I discovered that the Pobjoy
R was made under licence by Walter as the Walter Mira.
an email to Walter caused some searching among the old retired
employees and one was able to confirm that the tooling and all the
drawings were destroyed in WW2 when the factory was the target of an
allied bombing raid.
I found the pictures in google image searches.
google image on "walter mira engine"