If you read Ruth's volume two
"War Baby Comes Home", he points out that as many as 32,000 IP marked receivers may have survived. And he further states that IP are far from being the rarest carbines.
Specific things are hidden in the details
to expect Ruth to find each interesting point and show a person the way to find it is silly. The books speak for themselves; they are a lifetime's work and a very good read as well. The first step to understanding the books is to read them, repeatedly and form the index in your head. I find them very entertaining as an open and read book as well. Ruth's works are far superior to anything else on the subject.