speculative based on these Moore's from Cherokee Co, AL -
bev242 added this on 19 Oct 2011
"I think that M may stand for McReynolds my reason for thinking this is. I am pretty sure his father was James Moore & his mother was Mary [Polly] McReynolds. I found this on a census record's in DeKalb co. AL. James was in CHEROKEE CO.AL with Sam. Also I recently read that James owned a store in Cherokee which fits in with Sam being a merchant. Two McREYNOLDS boys were in the James MOORE household."
Home in 1850: District 24, De Kalb, Alabama
James Moore 62
Mary Moore 53
William M Moore 20
James J Moore 24
Joseph Mcreynolds 21
William Mcreynolds 18
Leonadas Mcreynolds 13
[ James nextdoor to Samuel Moore and Alexander Moore ]