This is the
sign for the golden number 18 (also written )
used in the Scandinavian clog almanacs. The golden number indicated
the cycle of the moon in relation to the number of days in a solar
year and made it possible to calculate the days of the full and new moons, as well as
the dates of eclipses. The golden numbers on the clog almanacs are
what is left of the old Nordic lunar calendar based on the phases of the
moon. With the arrival of Christianity the calendar was changed and
restructured according to the solar year. During the Middle Ages,
however, the peasants used the clog almanacs which combined the old
lunar calendar and the days of the new solar year.
This sign, by the way, was one of the letters of the alphabet used around the year 1000 in Rumania. It might be that its appearance in Scandinavia was a result of the Vikings' eastward commercial relations and journeys.