You've probably already been bedeviled by the atrocious name spelling on old census pages, and this is another example. Marcus was obviously misspelled as Markus, and "Mark" was commonly used as a nickname for men named Marcus.

The Marcus A. Looper who served in Co. H, 19th (Dawson's) Arkansas Infantry, was born in 1843, and died in 1922, and is interred in Union Ridge Cemetery in Sebastian County, Arkansas.