The T-rivets would appear do appear to be iron to me. I've always been puzzled by the exact means of T-rivet construction, in this case they appear to have been trapped in the channels by pushing the edges of the channel over the head of the T-rivet. This was a funeral pyre burial and if I remember many of the artifacts, like the maille shirt, show evidence of melted copper. Perhaps why much of the hilt decorations like the twisted wire and much of the overlying inlay is missing. It must have been stunning!