German soldiers on outpost duty near Antwerp, sharing their food with Belgian orphans, published in 1915.
The basic Pickelhaube, as seen in the photo, was made of hardened (boiled) leather, given a glossy-black finish, and reinforced with a metal trim. Starting in 1892, a light brown cloth helmet cover, the M1892 Überzug, was issued for use during manoeuvres and active service. The Überzug was intended to protect the helmet from dirt and reduce its combat visibility, as the metallic fittings were highly reflective. As you mentioned regimental numbers were then sewn or stencilled onto the front of the cover.
This photo was most likely staged to generate evidence that countered Great Britain’s aggressive propaganda campaign against the German occupation of Belgium. Given this assumption there would be little reason for these men to don their Überzug.
As the war progressed, and Britain’s blockade limited Germanys leather supply, the economic factors you mentioned drove the government to produce Pickelhauben from thin sheet steel. However by 1915, as demand rapidly outpaced supply, pressurized felt and even paper was used to construct pickelhauben
By 1916, the Pickelhaube was slowly replaced the the new Stahlhelm (steel helmet) which offered greater over-all head protection.