The original s/ns were routed/engraved. An engraver should be able to duplicate this if he had tooling that was hard enough to work in steel, rather than softer metals like brass or silver.
Alternatively, the s/n could be stamped, but this wouldn't produce the same fidelity. Once you locate a set of stamps with approx the same font size as the original markings the trick would be to make up a brass spud to fit the chamber so as to avoid any distortion which might be caused by hammering on the stamps.