I believe it was based on the qualities that the water used in the media would leave. They know what each signature is.
When I worked in the lab, we used distilled water (ALWAYS!!!) when making the media. That was part of my job: replenish the stock of media. One would never use tap water.
So if it is true that you can trace where it was made by tracing the source of the water....then you have to say that the water used in the media was bottled somewhere in the N.E.
One — that’s a general answer to a different, but related, question. It doesn’t answer what method was used, nor what signature was the key, nor how strong a signature it was.
Two — dried spores, how much water do they have?