Notice the word patent stamped on your whistle ?? What does that mean and how does it work ??
Firstly, patents are applied for and granted. They also have a duration. Early on they lasted longer. Why is that helpful ??
Well, if a whistle has that stamp, then you know what years it falls into right ?? But not always is it that easy . For example you often see on scout whistles by Hudson the stamp patent and also the number Rd. No.540164 and that pertains to the registered design of the pillar top that they used almost exclusively on their scout whisltes. However the patent would be for whatever was patented, the two piece top dated 1908 or the diaphragm dated 1898, or whatever it was for. That is important for your starting point.
This dates the whistle to either 1898 to 1912 or 1908 to 1922. You remember that the patent at that time was good for 14 years– this was set because of the length of two apprenticeships and continued to be so till 1919. Then it was lengthened to 16 years and later even further to 20 years.
The point being that if you found any circa 1900 whistle with a stamp of patent on it you could safely assume that it was dated from that patent date to 14 years later. However, don't confuse it with registered design dates.
So the next thing to do is look closer at the construction to date its start.