This story appeared in the March 1931 issue of “Far East Adventure Stories“. This pulp title published at least three Foreign Legion tales during it’s short run of twelve issues. In addition to this story there was “The Affair at Ain Hadoun“, also by Theodore Roscoe, which appeared in the first issue (October 1930) and according to the ad on the last page of this story there was Cadets of Gascoyne by Jacland Marmur. There might be more but from looking at the story titles it is hard to tell. Six of the twelve issues of Far East Adventures are available at Adventure House.
The Last Outpost is not one of Theodore Roscoes’s Legion stories featuring Thibout Corday which he was wrote for Argosy. Like many pulp stories this one is a mystery story in an exotic locale. The garrison at Fort Avant-poste is wiped out but by whom? Was it a renegade American officer (Brown) that seems to have vanished from North Africa or is it something more sinister? In any case, it’s hard for the other American Legionnaire (Smith) to find the answer when the Tuareg tribes keep attacking. Lots of action perhaps to mask a somewhat weak plot.