The story
Whistles first appeared in a Dixon catalogue in 1883 though some of their whistles appeared in gun, rifles & sporting goods catalogs of American manufacturers & distributors as early as 1872 for [dog call]s made of Britannia metal,
Early models of Dixon horn made Round pea whistle were in use with carriage drivers & railway conductors since the 1840s and mailmen of the Post Office as early as 1850.
From Wikipedia’s article on James Dixon & Sons, shared under CC BY-SA 4.0.3