Luckily no one is asking you to guarantee the authenticity of one of the finest & largest collections of art & artifacts out there.
Gilcrease was a multi-millionaire oil man during the depression, there wasn’t much he couldn’t afford. (The other signed Declaration of Independence, having Greg Perino lead a permanent crew of excavators digging most major sites, a staff of 20 archaeologists digging in Central America, etc.) It is likely that there are some replicas added after Gilcrease passed away in the early 60’s, but it’s still an amazing collection.