^Hogg, Ian (2002). Jane's Guns Recognition Guide. Jane's Recognition Guides. Glasgow: Jane's Information Group and Collins Press. ISBN978-0-00-712760-3.
^Bruce N. Canfield "Desperate Times: The Liberator Pistol" American Rifleman August 2012 pp.48-51&83-84
^Eger, Chris (April 20, 2020). "Heckler & Koch's 1st Handgun: The HK4". Guns.Com. Retrieved February 23, 2024. Hitting the market in 1964, the HK4 greatly resembled the old HSc, but used a light alloy frame and plastic grips--which kept its weight down to just 18-ounces-- rather than the Mauser's steel frame and checkered wood grips.
^Kinard, Jeff (2003). Pistols: An Illustrated History of Their Impact. Bloomsbury Publishing. p. 267. ISBN9781851094752. Manufactured between about 1972 and 1984, the Heckler & Koch Model P9 and its military and police-type counterpart, the P9S incorporate numerous modern synthetic components.[...]The company introduced a caliber .45 ACP model in 1977.