Although google seems to mainly throw up exactly this story hundreds of times when looking for hair tress, I found this article:
What the archaeologists did not expect was, as the skeleton was being cleaned in the laboratory, the discovery of a pair of gold hair tresses inside the man’s jaw. The new hair tresses were in the same style as the Archer’s.
Only half a dozen finds of these earrings have ever been made before in Britain, so it was remarkable that the graves of two men with these symbols of power were found side by side.
Surprisingly, from the context of this article, it seems that hair tress does not refer to something worn in the hair, but it's actually an earring, presumably shaped like a tress of hair.
Well, actually, digging on on that page, I found this:
Some archaeologists think that they were worn around the edge of the ear. Others think that they were worn in hair braids.
So it seems to be a name (coined by archaeologists?) for a piece of jewellery that may have been worn either on the edge of the ear of on a hair braid. It seems this usage is not common enough to show up in (m)any dictionaries... maybe archaeologists should publish more!