“I cant see that the chemicals themselves could be ‘fingerprinted’ back to the source”
But IF Porton has managed to obtain samples of the susbstance, they ought to be able to detect the contaminants that indicate how it was made, which chemical synthetic route.
The original USSR labs were in Usbekistan or Kazakhstan, I think, & were decommissioned by the US after the Soviet break-up. They were reported to have identified the methods used to make the V-agents. I guess that’s where Porton obtained their samples for comparison purposes, if they have any.
Alternatively, Porton might just have IDd the agent using normal analytical methods, and not have any of its own.