Bracketed letters in a single word

[frogboxer]

What do the brackets indicate when they are placed around letters in a word? I see this in extremely old newspaper articles. I can't find any explanation in style guides that would explain or condone such usage of brackets.

Example:

I was hunting in the mountains about twenty mi[l]es south of here ... and when I returned to [c]amp, I saw ashes and charred s[t]icks from the fireplace had been scattered abot.

Thank you.

 

[Rover_KE]

The letters were missing from the original text and the newspaper editor added them to make the meaning clear for readers.

Rover