Belgium made copy of the British Mk.II helmet, made from 1949 - 1952. ABL is Armee Belge/Belgische Leger which makes this the Army version (Based on color and the ABL stamp). Prices seem to be around the $40.00 range plus or minus, depends on how bad the person wants it!
ABL = Armee Belge/Belgische Leger (Belgium Army in French and Netherlands, the languages spoken in Belgium)
1949 = The model (First made in 1949 hence the model number)
X.B. = Xavier Buisset, Vilvoorde, the manufacturer
1951 = The year of production