From my own experience in documenting the military buried in Riverside -- there are two A. S. Martin stones, one in this Block, Block 12, and one in the G.A.R. Military Section, Block 27, Lot 1. This one in block 12 is for August Martin who died in 1908. (The stone really does not belong to him, but it is here). These men are not one in the same.
See below:
A S Martin, adult, interred: 19 Nov 1900 in Block 27, Lot 1. Where as: August Martin 27, was interred: 15 Apr 1908 in Block 12, Lot 5. https://history.denverlibrary.org/sites/history/files/Riverside_Cemetery_Burial_Registers_1876_1963.pdf