The Roman numerals were I, II, III, IV, which represented which quarter of the year the watch was produced.The numbers simply represented the two-digit year in which the watch was produced.
ABOVE: Around 2005, Rolex started engraving the Case's Serial Number into the inside edge of the case (at the 6 o'clock position, under the crystal and below the bezel), also known as the rehaut, as shown in these images, which were taken at BASELWORLD 2005, in Switzerland in early April.
While it was originally believed that this resequence started over at 100,000, or even 20,000, but evidence suggests that it actually restarted at 10,000 or possibly lower.
Around this same time, Rolex had initiated another dating system--They stamped the inside of the case back with a code consisting of a Roman numeral and two numbers.
Therefore, the code "III 53" would represent the 3 quarter (i.e.
July-September) of 1953--this code continued until around 1970.
Therefore, letter prefixes could end at random times of the year, and did not start or stop at the beginning or end of a calendar year.