Except they had to do it all old-school: no modern databases, no quick lookup and correlation of geographic info, no optical character recognition.

And they also kicked in doors and murdered people routinely.

Which funny enough is also happening in the US, but has nothing to do with mail tracking and everything to do with approval of no-knock police raids and general apathy to ever doing something about obvious corruption and brutality by people like Arpaio.

Who in turn has absolutely no access to any of said surveillance infrastructure to start with...

Hey, someone's got to stand for freedom and democracy, right? Now let me rendition you to a situation of torture and solitary confinement without trial.

But a serious amount of tech for their time and era/system. They had industrial scale rooms with steamers to open up envelopes and the likes.

Also the great east berlin telephone line distribution node: 50 wires coming in, 100 going out ... only 50 of those were for households.

All of this can be seen in the Berlin Communications Museum.

The DDR use to steam open all the mail, we've not yet come to that.