I confirmed with the person who submitted the question (see previous post) that this monument, located at Asbury United Methodist Church on Grand Avenue, is in fact the one she was looking for; she said she remembered it from the vicinity of 45th Avenue West and First Street.
It’s a big granite boulder with "Oneota" carved at the bottom. The plaque recognizes not all the Merritt brothers, but instead one of them – Rev. Lucien F. Merritt. Here is a closer view:
Now, given some more time, I’d look into just how this monument made its way from its old location to its new location – but I’m short on time right now. I assume there is some connection between Asbury United Methodist and the church mentioned on the plaque. If anyone wants to fill in the gaps, please post a comment or e-mail me at firstname.lastname@example.org.