The # 25 is Hannah's 1985 bike. He was the only rider to run a rear drum that year, everyone else had a disc and the arm on ebay has no mounts for a disc brake. In 1986 all Honda's ran production swingarms with rear disc mounts, so can't be that either.
I doubt it's '84 as I think they had a right hand side drive, so that rules that year out.
I also can't see that weld just behind the swingarm pivot on any of the works bike pics, that the ebay arm has.
So it's either an arm from Hannah's bike or a mock up, and I'm erring towards the latter, considering it's only got about $150 worth of bids, most people prob think the same.