EDIT Alternative Roman Numeral 1000 This character was used to represent the number 1000 even though there was another and more common representation: the letter »M« (U+216F Roman Numeral One Thousand).

This is a ligature of the Latin capitals C and D (for 100 and 500 respectively). Since the printers of the early letterpress times (16th century) usually didn't have this ligature it was often replaced with CIↃ (see pictures).

The reversed C (= Ↄ) can be found at U+2183 Roman Numeral Reversed One Hundred.

 jsw