Prudent to assume that the large majority of what is in a received Fiume collection is forgery - they were forged right from the start - base stamp, overprint, or both. Lots of investigation fun to be had for sure :)
A couple additional resources to what rog mentioned -
http://www.fiume-book.net/Fiume-Forgeries.html From website of Ivan Martinas, expert on Fiume stamps (full details on types & forgeries are in the catalogue he wrote, excellent publication)
https://stampforgeries.blogspot.com...18-1924.html