It was common practice by the firearms factories to save frames and other parts that came through production and finishing with flaws in them by filling those surface flaws with braze. The surfaces then repolished to level the imperfections.
Then the pistol would be given a nickel plate finish which would nicely cover both the steel base metal and the braze filler.

That was then,,black powder days and all that. Perhaps very early smokeless powder use.
On a modern firearm, that much heat overall is frowned upon but I have seen it done.

Low temp hard solder (used to commonly be called silver solder) is another option. Some flow at around 1000F+/-.
Still alot of heat to play onto the frame, but less than brazing temp up at around 1600F.
You can filll with one of the new tin/silver soft solders. They melt at around 400F. They take plating very nicely just as brass filler does. Just not as hard of a surface as brass. I've used it before for this very reason on small parts and it works well.
This is the same idea as using lead as already suggested as a filler.
The silver/tin soft solder will take plating though where as lead/tin solders or plain lead filler won't generally.
All this braze, hard solder and silver/tin soft solder stuff is of course if you are considering a plated finish to recover the revolvers looks.

I'd take it down as far as possible to remove the pitting. Any left I'd fill with tiny steel inlays, but I do that all the time anyway.
The other method generally used is to dot those small pits left over with TIG or Laser welding and then complete the polishing.

Just some more options as they say..