Background : When did 'Last Cowboy' die ?

[Racer]

Trying to help a GM friend with this information , but web searches are conflicting & don't give dates .

Current games involve investigators on trails of two lost locations in North America - think Lost Dutchman Mine , McKenna's Gold or Valley of Gwangi type discoveries .
One is near the West Coast and stumbled upon by group of cowboys & a priest during a heavy snowstorm in late 1800s . They return after spring thaw & can't find correct area .
Fast forward to modern day , where documents , detailed notes by the priest & cine film of last survivor giving testimony are presented to players .

Timeline becomes very important as whether this individual would realistically live into the Space Age & satellite imaging of landscapes etc . Topography of land features is crucial to game plot , but drawing together these separate plot devices is tricky .

Game is online due to lockdown & players are smart . GM didn't want to look like chump if a quick google by them derails the whole plot . Thus he asked me for ideas & an help/advice/suggestions .

Game is Black Ops/Cliffhangers/Tome Raider type , with elements of Da Vinci Code & Nation Treasure thrown in .

So has anyone got ideas on this ? Plus any other ideas to improve game plot & overall campaign setting will be very welcome . Trying to avoid well used clichés to avoid scenario feeling stale .

Thanks .