discs of a particular lift, and allowed for traceability in case the helmets exhibited defects. The "lot and lift" number is in reference to the time when the fabricator received the helmet discs, not when they were made into finished helmets. Lifts of heats were not loaded onto or unloaded from railcars in any particular order, and were often warehoused (also in no particular order) before being finished. The helmet discs were drawn to a depth of seven inches to create the rough helmet shape, or "shell," and the edges were trimmed. The edge of the shell has a crimped metal rim running around it, which provides a smooth edge. This is usually known as the "rim". The rim has a seam where the ends of the strip meet. On the earliest shells the seam met at the front. This was moved to the back of the rim in November 1944 At this time, the rim also went from being made of stainless steel to manganese steel. On each side of the shell, there are stainless steel loops for the chinstrap. Early World War II production shells had fixed, rectangular loops, and mid-war to 1960s helmets feature movable rectangular loops. This feature was adopted in 1943 to address the problem that when earlier helmets were dropped, the fixed loops were more susceptible to breaking off. Early shells for paratrooper helmets feature fixed, D-shaped loops. The shells were then painted with flat olive drab paint, with the paint on the outside of the shell sprinkled with either finely ground cork (World War II era) or silica sand (postwar). World War II-production helmets feature sewn-on cotton web olive drab shade 3 chinstraps, replaced gradually throughout 1943 and 1944 with olive drab shade 7 chinstraps. 1950s and later production chinstraps are made of olive drab webbing attached to the loops with removable metal clips. Nylon chinstraps were introduced in the U.S. military in 1975. These straps featured a two-piece web chin cup and were fastened by a metal snap rather than buckle. Uses Many soldiers wore the webbing chinstraps unfastened or looped around the back of the helmet and clipped together. This practice arose for two reasons: First, because hand-to-hand combat was anticipated, and an enemy could be expected to attack from behind, reach over the helmet, grab its visor, and pull. If the chinstrap were worn, the head would be snapped back,