The Festungsflammenwerfer or Festungsnahkampf Gerät was a ferocious weapon which was built only two times in the Atlantikwall. Ba 70 Alex posted a report on the French Atlantikwall forum.
The Germans built an extensive tunnel system with bunkers (120, M 152) on the Plateau de l’Atalaye in Biarritz. The positions, designated Ba 39 and 40, also housed the two fortress flamethrowers. A unique construction and finally there are some clear photos online. More info on the FN Gerät in English. In Panzerwerk 720 of the Oder-Warthe-Bogenlinie an FN Gerät remains.