Question: What style of helmet pre-dated the distinctive design used by the German army, a pin of which was given to First World War veterans by the Nazi Party? 

Context paragraphs:
Sozialverband Deutschland: The Sozialverband Deutschland (Social Association of Germany) is a German socio-political advocacy organisation.  It was founded in 1917 with a focus on assisting veterans of World War I as the "Bund der Kriegsteilnehmer und Kriegsbeschädigten" (Association of War Veterans and War Wounded), in 1918 became the "Reichsbund der Kriegsbeschädigten und Kriegsteilnehmer" (Imperial Association of War Veterans and War Wounded), and in 1999, after several further changes of name, became the Sozialverband Deutschland, with the note that it was founded in 1917 as the Reichsbund.

The Honour Cross of the World War 1914/1918: The Honour Cross of the World War 1914/1918 (German: "Das Ehrenkreuz des Weltkriegs 1914/1918" ), commonly, but incorrectly, known as the Hindenburg Cross was established by Field Marshal Paul von Hindenburg, President of the German Republic, by an order dated 13 July 1934, to commemorate service of the German people during the First World War.  This was Germany's first official service medal for soldiers of Imperial Germany who had taken part in the war, and where they had since died it was also awarded to their surviving next-of-kin.  Shortly after its issuance, the government of Nazi Germany declared the award as the only official service decoration of the First World War and further forbid the continued wearing of German Free Corps awards on any military or paramilitary uniform of a state or Nazi Party organization.

Werner von Clemm: Werner C. von Clemm (27 September 1897 - 14 April 1989, Biddeford, Maine) was born into a German family that married into German nobility, the Nazi party and prominent American families.  His father-in-law was a Vice President of Citibank.  In 1922 he immigrated to the United States.  Prior to this he had served in the German army during World War I.  He was a representative of the German international banking firm Hardy & Co. and an employee of the Pioneer Import Corp., which imported materials from Germany and handled his cousin and Nazi diplomat, Joachim von Ribbentrop's champagne business.  Along with his twin brother, Karl he had become friends with U.S. oil man, William Rhodes Davis.  In the United States Werner became Vice President of Davis' company Davis & Co., which shipped large quantities of oil to Germany and became a propaganda strategist and financial aid of the America First Committee, which encouraged American isolationism.  In 1940 the Nazis seized large quantities of diamonds from Belgium and the Netherlands, which Werner was smuggling to the U.S.  The United States was neutral and did not declare war until December 1941.  To avoid detection he moved the diamonds through various European cities, shipped them to South America and they eventually found their way to New York City.  The operation was stumbled upon by mistake by agents of the Treasury Department when they searched a Budweiser beer box being carried by two American soldiers of German descent.  On inspection of the box they found it had a false bottom under which were one hundred letters and government bonds.  The letters detailed Werner's diamond smuggling operation.  After further investigation von Clemm was arrested on 28 January 1942 and was sentenced to two years in prison for his crime.  On his release he entered private banking.  He was also an agent of the German intelligence agency, Abwehr and was involved in espionage during World War II.

World War Veterans: The World War Veterans was established in 1919 as a progressive organization of American veterans of the First World War.  It officially claimed to have been organized just 9 days after the armistice, on November 20, 1918 in Bois, France.  The group filed formal certificate of incorporation in New York state on February 13, 1919.  The purposes of the corporation stated at that time included (1) To band together veterans of World War I who have served in any branch of the United States service; (2) To preserve the ideals for which these veterans served; and (3) To aid such veterans to secure adequate employment and to facilitate their return to civilian life.

Otto Telschow: Otto Telschow (27 February 1876, Wittenberge, Brandenburg – 31 May 1945), a German Nazi Party official, was born in Wittenberge and became a police official in Hamburg.  Telschow joined the German Social Party in 1905.  In 1925 he joined the Nazi Party, and was the founder of the regional Nazi newspaper, the "Niedersachsen-Stürmer".  In October 1928, Telschow was appointed Gauleiter (regional party leader) of the Nazi party's regional subsection Gau Eastern Hanover, a post he retained until the end of World War II.  Telschow gained more influence after 1935, when the Nazi-party "Gaue" usurped the functions of the streamlined German states.  In 1930 he was elected to the Reichstag for the Ost-Hannover electoral district, and remained a member until 1945.  He was taken prisoner by the British Army at Lüneburg and committed suicide in prison by slashing his wrists.

Bill Stone (Royal Navy sailor): William Frederick "Bill" Stone (23 September 1900 – 10 January 2009) was one of the last surviving five First World War veterans who served in the United Kingdom's armed forces and one of the last surviving two seamen worldwide, along with Claude Choules.  They were also the last two to have also served in the Second World War, although Stone saw action only in the Second World War as he was still in training when the First World War ended.

Gottlob Berger: Gottlob Christian Berger (16 July 1896 – 5 January 1975) was a senior German Nazi official who held the rank of "SS-Obergruppenführer und General der Waffen-SS" (lieutenant general), and was the chief of the SS Main Office responsible for "Schutzstaffel" (SS) recruiting during World War II.  Serving in the German Army during World War I, he was wounded four times and awarded the Iron Cross First Class.  Immediately after the war, he was a leader of the "Einwohnerwehr" militia in his native North Württemberg.  He joined the Nazi Party in 1922, but lost interest in right-wing politics during the 1920s, training and working as a physical education teacher.

Nazi Germany paramilitary ranks: National Socialist paramilitary ranks were pseudo-military titles which were used by the Nazis, represented by the Nazi Party, the National Socialist German Workers' Party ("Nationalsozialistische Deutsche Arbeiterpartei"; NSDAP), between the years of 1920 and 1945.  Since the Nazi Party was by its very nature a paramilitary organization, by the time of the Second World War, several systems of paramilitary ranks had come into existence for both the Nazi Party itself and the various Nazi paramilitary organizations.

Steel Helmet Veterans Memorial Badge: The Steel Helmet Veterans Memorial Badge was a pin issued by the Nazi Party to veterans of the Stahlhelm veteran association of the First World War.

Stahlhelm: Stahlhelm (plural "Stahlhelme") is German for "steel helmet".  The Imperial German Army began to replace the traditional boiled leather Pickelhaube (spiked combat helmet) with the Stahlhelm during World War I in 1916.  The term "Stahlhelm" refers both to a generic steel helmet, and more specifically to the distinctive (and iconic) German design.

Supporting paragraphs: Steel Helmet Veterans Memorial Badge Stahlhelm
Answer: boiled leather Pickelhaube