MYSTERY SOLVED
Here is the full answer to the mystery of the polaroids: I bumped into Ken, the owner of the shed, some weeks ago and asked him about the photos. He works for a firm that used to export reproduction furniture to the United States. They took two photos of the items in each consignment, one for the buyer and one to keep as a document which they stuck on boards. When they stopped exporting, he used these boards to finish the shed.
