EDIT Roman numeral In early letterpress printing, the reverse C in combination with C and I was used frequently to form an imitation of the ligature ↀ (U+2180) that represents the number 1000, because the early printers didn't have a cast type for this character.

Since there is already a representation of the number 1000 (the letter M, U+216F Roman Numeral One Thousand), there are only speculations as to why this ligature was used in the first place. A theory says that is an imitation of the Greek letter Φ (U+03A6 Greek Capital Letter Phi) that also stands for 1000 and would be missing just like the ↀ amongst the first cast metal letters.

 jsw