Easily done.

Back in the previous century when men were Men (and women were not Men) camera lenses were set in metal and distance, aperture and related markings were engraved and filled with contrasting paint. With age the paint either fades or falls out, making for a hard to use and unattractive tool. (Modern lenses use plastic, with awful screen printed numbers. Once these wear off you are stuck).

The $13,000 Leica Noctilux of today.
Yes, they still use paint in the engravings.

I have had this issue with Leica lenses and more recently with a tool in my garage, a low range Husky torque wrench which covers the 20-200 inch pound range (1.7 to 17 ft.lbs. or 2.3 to 23 Nm). The paint in the vernier markings on the rotating barrel had completely disappeared, testifying to years of use of this wonderful tool with low torque fasteners on those relatively soft alloy cases on BMW motorcycles. This made it very hard to set torque in the relatively low, non-directional light in the garage. Easily over-torqued, once you strip a female thread in one of these engine cases you are in a world of hurt. And that’s easily done without the proper tool. Forget doing it by ‘feel’. ‘Feel ‘ is for lovers, not mechanics.

The process of restoring those markings is simple.

The affected part is thoroughly degreased using isopropyl alcohol.

The fill in paint is generously applied in all directions.

The paint is wiped off with a rag, leaving filled in, engraved numbers.

Amazon carries the paint sticks in a variety of colors. Before use, be sure to shave the end off to reveal fresh paint, as the surface layer will have dried, and store the stick in an air-proof baggie to extend its useful life. And if torque wrenches are your thing, store them un-tensioned or prepare to deal with the consequences.