From the wording it sounds like the monument is a GAR monument placed and/or dedicated to E.P. Case Post 372. In this case, the name of the post is E.P. Case Post #372. Each post was named after a person/battle/etc. associated with the Civil War.
So most likely a GAR monument, not a monument for E.P. Case.

Mahlon Erickson
Graves Registration Officer
Department of Oklahoma
Sons of Union Veterans of the Civil War (heirs to the GAR)