They all had a circle, and 'Pietersdorp' in broad black letters.

But this envelope had nothing except the stamp.
I was still slow at detective work, and it was some minutes before the explanation flashed on me.

The letter had never been posted at all.
The stamp was a fake, and had been borrowed from an old envelope.
There was only one way in which it could have come.

It must have been put in the letter-bag while the postman was on his way from Pietersdorp.

My unknown friend must therefore be somewhere within eighty miles of me.