another thing the whistle books are great for ?? They are copyrighted so there is information that is only in them.
For example — UK makers names indexed. in CPWs ( a few USA )
History in each maker that is original and copyrighted research.
Lists of information in CPWs like patents that have been accessed from Hudsons files and is also copyrighted.
Especially nice is the lising that is quite large of STAMPS that different makers used in the back of CPWs. A very nice research tool.
Years of makers business that has been researched out by the author.
Each whistle book has its info, but especially researched through Hudsons is CPWs.