The 21 refers to the SCHED column on the tag. This means the body was constructed on the 21st day of production with the 59 denoting it's order on that day's list (59th in line).
That said, what I mentioned applies to regular Kaiser production - and that for 1954 is reported to have started about October 21, 1953. Since Darrins were constructed and assembled at a different site with a different production schedule, you probably cannot count the days from the 21st. Instead, you'd need to know when Darrin production started and how many days a week the Jackson plant operated.