They have traced serial numbers on pieces of hardware and on weaponry. That doesn't equate to circumstantial evidence to me; it equates to physical evidence.
Do I need to be corrected on the difference between the two?
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.