What I don't see explained is how Union Commerce Bank came to possess all these historic checks from banks all over the country.
Yes! Like you said, the supposed “checks in their records” aren’t even FROM that bank; they’re from all over the country.
Something’s definitely fishy.
“I smell a rat.”
It’s not a rat. They inherited the Union Commerce material and bought the others from collectors like this crew:
http://www.ascheckcollectors.org/index.html