One of two ways for marking them permanently. Neither way offers ID from casual observation without looking directly at the floorplate.
1) Use an electric engraver to make any ID mark you want anywhere you want. I prefer the floorplate. If you do it real carefully, they will look good. If you want contrast, you can use the correction fluid or fingernail polish (color is your choice) to fill in the engraving (spread some on and wipe off, quickly). Or,
2) You must have a piece of really hard and smooth steel as an anvil surface to do this one. If the surface is really smooth, you can use the anvil part of a vise. I use a section from an old truck leaf spring. It
HAS to be very hard to prevent distortion of the floorplate!
Remove floorplates from mag. You can use either a manual (requires small ball pien hammer) or an automatic (like a spring loaded center punch) alpha/numeric die type set. I prefer the manual one as I can better control the die. Stamp in the markings of your choice. Again, if you want to contrast the stamped markings, use the contrast fluid of your choice.
One nice thing about using the floorplates is that if you screw up, you can fix it cheap.