the biggest smoking gun is that the idea that immediately upon arriving un America, Scrooge helped out his riverboat-captain Uncle with the original “Great Steamboat Race”. It's often cited as one of the apparent lifts from DuckTales to Rosa, which would belie Rosa's claim that he never watched Once Upon A Dime; but it's actually a Chalker invention with both the cartoon and the L&T poached independently.
Isn't it an obvious idea? In two Barks stories it is said that young Scrooge worked in a riverboat, and in another Barks story it is said that Scrooge had an uncle who was a riverbot captain. Since Scrooge had to leave Scotland for America at an early age, probably before the age of majority, it seems logical that a barksian biography would have Scrooge go live with his uncle and work with him to learn the job. Plus, while I think the DuckTales version implies that Scrooge partecipated to the original "Great Steamboat Race", in the Rosa version the original race is described as having happened 10 years before, while what we see is a separated race.
Also, about Scrooge earning his Number One Dime, I'll quote what I wrote in other threads more than two years ago:
Speaking of character designs, young Scrooge from this 1969 Italian story is not much different from Rosa's version. Evn the plot itself, with Magica travelling back in time to get the dime and Scrooge earning it by shining shoes, is basically an ante litteram version of "Of Ducks and Dimes and Destinies". The explanation for why he earned an American coin is different, though.
After it was mentioned on another thread, I now also want to see this Scarpa story whose plot is reportedly similar to Rosa's Of Ducks, Dimes and Destinies.
It's not a Scarpa story: the writer is Rodolfo Cimino and the artist is Luciano Gatto. And yes, the plot of this 1969 story it's similar to Rosa's Of Ducks and Dimes and Destinies.
Another Italian story that I heard is similar to Of Ducks and Dimes and Destinies is this one from 1975, though in this case I can't confirm the similarity since I haven't read it.
Finally, this 2007 Italian story also features Magica travelling back in time to get the dime, though in this tale she meets Klondike Scrooge rather than kid Scrooge. This story is based on Barks and Rosa, though the similarity to Of Ducks and Dimes and Destinies is coincidental since the writer said at the time he hadn't read that story yet.
The last story of course has since been published in the USA early this year. It's not about Scrooge earing the dime, though there is a flashback of 10 years old Scrooge earning it.
About the "riveboat coincidence", Rosa also commented on it in a 2010 Papersera message, albeit in a different context:
But no matter who wrote it, the matter suggested above can be easily explained. Barksian fact: $crooge went to America at an early age to find work. Barksian fact: $crooge participated in a riverboat race in 1880. Barksian fact: $crooge had an uncle who owned a riverboat and sometimes participated in races. It is therefore an OBVIOUS conclusion, and clearly makes for a more interesting story, that if $crooge went to work on a riverboat when he came to America, it would be on his uncle's riverboat. Anyone writing a biography of $crooge based on Barks' old stories would come up with the same idea without any help.
For the context of the quote, you can start reading from here.