yup its happening to me as well, and i have notified Playstaion via email about this issue as well so i might get a direct message from sony and Hellfire about getting the code to work.
At Tycoon I applied and in email received I got the code. Went to Store from PS3 and entered code and received invalid message.
Later I sent through my hand items in clothing and discovered I had already received sometime back the Pirate chainsaw from Hellfire from somewhere else (I don't remember where). IOW, I already had it which I hadn't remembered. I assume I'm talking about the same Pirate chainsaw as OP.
You've probably already got it and don't know it, since it's not a new hand item. They've been giving out the code for it for a little over a year (starting around 10/5/2011 I think) for signing up for their news letter.