I recently discovered the trait "Demonbane Summoner" from the Monster Summoner's Handbook: "Your line is derived directly from the god callers of Sarkoris. You adamantly oppose the demonic forces of the Worldwound in hopes of reclaiming your lost lands. The attacks of creatures you summon are treated as cold iron for the purpose of overcoming damage reduction." A Summoner's Eidolon is definitely a summoned creature, and this would work RAW, but is that the intent, and does this apply to the manufactured weapons of the Eidolon? What about weapons of things like a Hound Archon?
It doesn't specify natural attacks, just 'attacks' so weapon attacks should work just fine with the trait. This applies to both eidolons and creatures summoned via Summon Monster.