In response to my own post, I found the answer on the Afrigenius website in a message same subject dated 2004. The numbers are almost certainly tick-marks made by the census tabulators in Washington when they were compiling the statistics. These marks do NOT indicate runaways. The census clerks had to count the number of slaves classified as "Black" and "Mulatto" - and in most places the number of Mulattos was less and therefore could be counted more quickly - if they counted the total number of slaves, then subtracted the number of mulattoes, they would have the number of blacks without needing to count them separately. Tabulators sometimes marked the lines as they counted data, to keep track of where they were on the page. The number of slaves reported fugutive from the state was miniscule, which meant this column was almost always blank; being immediately next to the column for color made it a convenient place for tabulators to make tick marks when counting.