Most references I find are coming from the (excellent) work "A Separate Identity - Organizational Identity Among Readers Of Zion's Watch Tower 1870-1887" by B. W. Shulz and R. M. de Vienne (Fluttering Wings Press, 2014), but I've seen it in other places.
On a footnote of that book, there's this entry: "Age to Come is a biblical phrase. It was used early in the Millerite movement, but not always to Literalist doctrine. The term was not exclusive to Millerites and Literalists but being a biblical phrase was used by others". (p. 55)
Seems to me the authors are referring to churches that emerged from a subset of the Millerite movement (p. 56) BUT didn't evolve into Adventism?
Eden