The 124488 could have been inscribed by a watchmaker when the watch was serviced. Some people did that. Looks like another service record was etched inside the caseback below the 140 and also at the nine o'clock position.
Your grandfather seemed to love this watch and took care of it. You're fortunate to have this part of his life passed onto you. Something that was intimate to him and treasured.
May it always bring you fond memories.