I can recognise many of my scanned manuals. Even things like creases, damage and width of margins make them identifiable, before we consider other more subtle clues. There are plenty of things that can be moved slightly without affecting the accuracy.
I have contacted a few websites that have copies, some said they would remove them and did, some said they would but didn't, and some didn't reply or were uncontactable. With Radiomuseum I don't know how I could even check as I am not a member. In any event it would take ages to go through them all and provide enough proof to satisfy them. Generally it is more hassle than it's worth.
I paid a friend (as a contractor, he's self employed but was unable to do his normal work at the time) to do a lot of scanning for my new DVD-ROMs. We agreed a very fair rate, but that is still several thousand pounds I have paid out and won't see back until they are finished and I sell enough. People who steal work don't see or consider things like that.