I believe his prior service unit is: B Company, 1st Battalion, Mississippi Marine Brigade Cavalry -- this was a Union unit formed in Missouri.

If you go to the Missouri Digital Heritage website here:

http://www.sos.mo.gov/archives/soldiers/results.asp?sType=unit&txtUnit=1st+Battalion+Cavalry+Mississippi+Marine+Brigade&selConflict=Civil+War&offset=0

you will see a link for Harvey Edwards. Or, you can go directly to his record in this unit on the Missouri Digital Heritage website with this link:

http://www.sos.mo.gov/archives/soldiers/details.asp?id=S106293&conflict=Civil%20War&sType=unit&txtUnit=1st+Battalion+Cavalry+Mississippi+Marine+Brigade&selConflict=Civil+War&offset=0

and click on the "View Record" button to see this record:

Edwards, Harvey
Rank: Pvt
Captain: Brown
Enlisted: Jan 12 1863 in St. Louis
Mustered in: Feb 20 1863 in St. Louis
Remarks: Discharged Aug 23 1863 - Disability

1st BATTALION MISSISSIPPI MARINE BRIGADE CAVALRY (MO).

Organized at Jefferson Barracks, MO, January and February, 1863, for duty in Western Waters. Attached to District of Northeast Louisiana, Dept. of the Tenessee, October, 1863, to April, 1864. District of Vicksburg, Miss., to August, 1864.

SERVICE.--Action at Little Rock Landing April 26, 1863. Beaver Dam Lake, near Austin, May 24 and 28. Expedition from Young's Point, La., to Richmond, La., June 14-16. Richmond June 15. Grand Luxe, Ark., June 16. Expedition from Snyder's Bluff to Greenville June 29-30. Bayou Tensas June 30. Expedition from Goodrich Landing to Griffin's Landing, Cat Fish Point, Miss., October 24-November 10. Operations about Natchez December 1-10. Rodney December 17, Fayette December 22. Rodney December 24, Port Gibson December 26. Grand Gulf January 16-18, 1864. Expedition to Grand Gulf February 15-March 6. Lima Landing, Ark., February 22. Red River Campaign March 10-May 22. Fort DeRussy March 14. Worthington's and Sunnyside Landings, Fish Bayou, June 5, Old River Lake or Lake Chicot June 6. Indian Bayou June 8. Coleman's Plantation, Port Gibson, July 4. Port Gibson July 7. Consolidated with 1st Infantry, Missisippi Marine Brigade, August, 1864.

Battalion lost during service 2 Officers and 15 Enisted men killed and mortally wounded and 1 Officer and 38 Enlisted men by disease. Total 56.

Source of Data: "A Compendium of the War of the Rebellion, V.III" by Frederick H. Dyer, c1908, p.1302-1303

I hope this answers some of your questions.

Regards,
Kenny