Physical evidence is a form of circumstantial evidence. It stands in contrast to direct evidence (e.g. statements, documents, eyewitness testimony). Circumstantial evidence, technically, are simply facts that can be used to infer a conclusion - direct evidence expressly demonstrates the given fact (if believed).
Thus, finding Iranian weapons (serial numbers et al) are circumstantial evidence for Iranian involvement in Iraq, in that it makes Iranian involvement in Iraq more likely than without the weapons - however, that must still be inferred from the weapons. It's a fair inference, but an inference none the less.
I know. Lawyers quibble too much.
With the armor piercing weapons they had a sale of weapons with those serial numbers from Austria to the Iranian government that ended up in Iraq.
Isn't that...direct evidence? Or is that, too, circumstantial? (If so, why?)