This is a small ( it's 26 cm high) binocular periscope with graticule (on the right site), official service glass for the German Army. The designation H/6400 refers to the German graticule graduation and is typical for German army optics. This model was produced during the occupation of France.
It has still a sharp and clear view. Produced by S.R.P.I. = SOCIETE DE RECHERCHE ET DE PERFECTIONNEMENT INDUSTRIELLE à Puteaux (near Paris)
Adjustment of the interocular distance is made with a knurled knob between the oculars.
Weight: 900 gram, so it is easy to hold up for a short time.