There's a simple answer to this, apologies if it doesn't fulfill length requirements.

The Chinese Telegraph Code was in use before the first world war and is still used today. Both sender and receiver need to be in possession of a code book arranging the 10,000 most frequent characters by the four-corners system, each with a four-digit lookup number from 0000-9999. It's tedious, but still used to request certain diplomatic documents between certain countries.

You can see a scanned telegraph with transcription by hand here on Language Log with one person's account of its administrative use and range.

Not that this was necessarily the code employed by the dissidents you mention. One advantage of codes like this is you can personalize the code however you like by re-arranging the numbers or applying a cipher to them - so long as the receiver has the cipher. That would allow for secure messages.