All you want to know about the German combat helmet chinstraps.
The chinstrap for the German helmet as used during the Second World War appeared simultaneously with the introduction of the M1931 liner system. So around 1931, the design used in the First World War, two (sliding) buckles, was released, the new chinstrap had only one buckle. First it was of the roller type buckle, but the buckle was soon replaced by a normal one. The used material of the buckle was aluminum or steel.
The old roller type chinstraps stayed into service throughout the war. What also changed was the coupling of the chinstrap to the helmet. The chinstrap was no more attached with a quick release system but directly attached to the D-rings of the linerband. In order to ensure quality manufacturers were required to mark the fabricated chinstraps. One side of the chin strap was most commonly dyed black with the opposite side left natural. From 1937 we see that most straps are marked. Later (1942), the name of the manufacturer was replaced by a number. Each manufacturer had its own unique number, the so called RB or Reichsbetriebsnummer. From 1940, the majority of the produced buckles is out of steel with a layer of gray paint. Painting was done to prevent from rust and to reduce reflection.
Left or right
What side to wear the buckle of the chinstrap was first dictated on 14 November 1934 as part of Army Directive 122 describing the Model 31 helmet liner. According to a later order (HV 35, No. 691, with description in HV 36, No. 112) which was dated November 22, 1935, the shorter strap was to be attached to the left side of the helmet for those who are right-handed and vice versa. The reason is to prevent the buckle from getting in the way while operating the K98 riffle.