I cannot find a letter matching the filename "Letter to Karl Marx, December 6, 1867" in the input text.
The input text contains: 1. A letter fragment FROM Karl Marx TO Friedrich Engels (date unclear, likely December 6, 1867) - ending with signature "Your K. Marx" 2. Letter 301: FROM Karl Marx TO Friedrich Engels, dated 7 December 1867 3. Letter 302: FROM Karl Marx TO Ludwig Kugelmann, dated 7 December 1867
All letters in the document are FROM Karl Marx TO other recipients. There is no letter TO Karl Marx from any sender that matches the filename date of December 6, 1867.