Any presidential candidate seeking to overhaul a tarnished image would love to recruit the services of Sterling Cooper Draper Pryce, the fictional advertising agency in the TV series Mad Men.

Donald Trump appears to have done the nearest thing, finding a mysterious real-life agency called Draper Sterling.

The obscure organisation was the most intriguing entry in a set of financial documents filed by the Republican presidential nominee this week.

It received four payments, totaling $35,000, from the Trump campaign last month.

The payments were made on an American Express card for "web advertising".

Draper Sterling was not a name widely known in politics, or in any arena, and it had no presence on the internet.

Intrigued observers soon tracked it down, not to plush offices on Madison Avenue, but a suburban house in a quiet town in New Hampshire.