Photographic filters are produced within a sizing system called "series" that uses roman numerals, a numerical system that originated in ancient Rome, including the series IV with a 20.6mm filter size.
Starting in the 1930s, filters were also made in a sizing system known as a series mount. The series designations are generally written as Roman numerals, I to IX, though there are a few sizes not written that way, such as Series 4.5 and Series 5.5. IV 20.6 mm Roman numerals are a numeral system that originated in ancient Rome and remained the usual way of writing numbers throughout Europe well into the Late Middle Ages. 
