Riferimenti: [http://pencils.wikia.com/wiki/L%26C_Hardtmuth], [http://en.wikipedia.org/wiki/Koh-i-Noor_Hardtmuth], [http://it.wikipedia.org/wiki/Koh-I-Noor_Hardtmuth], [https://www.kohinoorusa.com/about], [https://collection.sciencemuseumgroup.org.uk/people/cp118887/l-c-hardtmuth], [https://forum.fountainpen.it/viewtopic.php?t=32043]
Riferimenti: [1], [2], [3]