The Caravan Crime by Fergus Hume — Read

This unexpected five hundred pounds had apparently fallen from the skies, and Dick stared at the bundle of notes--at the single line of typewriting--with a surprise too great to be expressed in mere words. This was a bolt from the blue with a vengeance, but how it had struck the intended person--himself--the young man could not imagine. With his mind full of the late interview with Mr. Bollard, he had failed to examine the envelope, but now did so, in order, if possible, to trace the sender of the money. How was it that this anonymous correspondent had learnt his new address? This part of the mystery proved easier to solve than he expected, for the notes had been addressed to Richard Lawson care of Constable Selwin, Sarley Village, with the instruction "Please forward."

There was nothing unreasonable about this. The criminal knew that Lawson and Selwin were deeply involved in the tragedy, and it was natural to suppose that they had kept in touch with one another. The packet had been posted from London to Sarley Village, thence back again to London. As there was no handwriting, no name, no registration, it would be difficult to trace the sender. Lawson flicked through the notes. They were all brand new, and genuine enough, as he saw. Dick rose to walk up and down his small room, very much puzzled, as he did not know what to make of the matter.