It's quite hard to write on / engrave into a cylinder

And the scribbling would have been done by someone wearing heavy gloves, probably while the rod was in a glovebox.

Plutonium is 2.5X as dense as steel. Also it's normally kept in a glove box so the workers are probably wearing thick gloves when doing the engraving.

Scribing the rods probably releases plutonium particles, a vicious carcinogen, as would more professional looking laser engraving. A Sharpie would be safer.