Hello Peter,
if it hasn't been said before then welcome to GMIC.
The fact that the RIC adopted the peaked/German/Staff cap in 1900 indicates that it was assuredly in use, unofficially, some years before that date.
The beauty of this forum is that these snippets of information come together to help us all form a more rounded picture of a given subject. I have said it before but better to have a group of knowledgeable people than one expert.
Cheers,
Stuart