As far as I can figure, it's the mold number. Parts have different numbers for different colours and printing, but that number you're looking at had to be part of the mold to get there, and I've found the same number molded into different colours of the same shape of part. I do keep track of any mold numbers I find on parts I own, but I only have a couple dozen numbers so I didn't make them searchable. Their only use, as far as I can see, is to confirm that a part with the same mold number as any given part will indeed fit into another as the original did. I don't know what the lettered compasses mean! I will make a note of the mold number you found, with thanks.