More ways than one to skin this cat.
I do a bit of work with building and refurbishing older wooden ramrods. On the older ones, that were even marked to show the load, they were made with a scratch from a knife. On the metal ones, a sharpie is fine if you can see it. For range work, I have two very good rods made by TC and Traditions. These and some field rods, I mark with tape wire markers, that are numbered. The tape sticks fairly well for a time. I have noticed some older wooden rods that have been marked with the Sharpie. On your in-line, you could cover up the old marks with Aluma-Black from Birchwood casey or equal. You could also remark with a white paint stick. On replacement rods and components, I'd look at Dixie Gun Works but there are others as well.
Be Safe !!!