Your ancestor's name and the letter he wrote just prior to his death are very familiar to me. I even quote part of his letter in a presentation I give occasionally on Gray's Brigade at Mansfield.

Booth shows "L. D." Lord enlisting at Camp Moore, which is an obvious mistake. The entry for Stephen D. Lord makes much more sense. My "educated" guess is that he and others from DeSoto went to Monroe to enlist in the 11th Louisiana Infantry Battalion, which was then stationed there and contained two companies from DeSoto.

One possible source for clues on the Mansfield Rifles would be the Natchitoches newspaper published during the war. It might contain some notice of the company passing through town on its way to Monroe. My recollection is that the paper is on microfilm either at LSU or Northwestern.