I've seen a few earlier mainspring barrels (with plates) in later Eagles. But they have always been restored/cleaned up/conserved examples. In other words, someone has worked on them "recently." I've never seen an unrestored, grungy Eagle numbered over 145000 with earlier style spring barrels.
Is it possible? I suppose so. Some Edison cylinder machines are known to have been assembled with unexpected earlier components as older inventory was uncovered. But after seeing hundreds and hundreds of Eagles over the past several decades and conducting an extensive survey of existing examples over the past 12 years, I've been struck by the "clean break" American Graphophone made between the first style Eagle and the later style.
I'm willing to share information I've found, but I'm not out to convince anybody. It's your machine, and you can believe what you like.
As for replacing Eagle mainsprings, yes, they're pretty straightforward. But if most collectors (including me) are restoring a B/BS with broken/weak mainsprings - but have a junk Eagle with good mainsprings, I'll bet they'd rather switch out the entire assembly (mainsprings/barrels) rather than go to the trouble, mess, and expense of replacing the mainsprings. I did exactly this with two Eagles on at least one occasion in the 1980s.
George P.