According to Collins' Historical Sketches of Kentucky: History of Kentucky, Volume 2 (found at GoogleBooks), by Lewis Collins, Richard Henry Collins, published in 1878, Granville Bowman was a State Representative from Cumberland County in 1816.
|Frankfort Argus, 26 April 1821|
We are authorised to announce GranvilleIt's hard to tell from the first image when Wm. Wood's term ended. (It reads 1814- )
Bowman, Esq. a candidate to represent the
counties of Cumberland and Wayne in the
Senate of Kentucky.
From GenealogyBank.com - Historical Documents results:
This document indicates that a contract was made with Granville Bowman - presumably to take care of these mail routes. It also notes that the previous person, Andrew Craig, failed to perform the service.
Granville may have been given this opportunity because, according to the Cumberland County, Kentucky USGenWeb site / Post Offices, Granville Bowman was appointed postmaster on March 3, 1828. (He only held the position for a couple of months.)
Interesting what you can find online!