I seem to recall reading in one of the early Han Solo novels that the blaster was Han's when he was still in the Imperial Navy, and the disc identified the blaster as Imperial property. Han kept the blaster after he was booted out of the service and pried off the disc in disgust for the Imperials. Which could explain the circular mark(or missing paint/finish). Although, it's been mentioned in several books that his blaster was custom-made; which could still be true, and the former-Imperial blaster was the base for his "custom-job."