Between 1950 and 1954, Zippo lighters underwent notable changes influenced by both design evolution and external factors like the Korean War. During this period, Zippo's iconic bottom stamp—used to date lighters—continued to evolve. Lighters from this era typically feature the word "ZIPPO" in block letters with a patent number below it. The original patent number 2032695 was replaced by "PAT. 2517191", which was issued in 1950 and used from 1953 throughout much of the decade. The Korean War (1950–1953) influenced Zippo's production materials. Zippos manufactured in 1952-1953 were steel instead of brass, as brass was prioritized for military use. These steel-case lighters are slightly heavier and have a distinct feel. Once brass became more available post-war, Zippo gradually returned to using it more frequently.