Yes, there is a seam on top that runs front-to-back. The tank may be made in three pieces; left, right and bottom. My tank had filler covering the top seam. The seam itself was not terribly well done but that's what the filter is all about. It was clearly original.
EDIT: The picture Groover shows should make it clear that at least three pieces are needed to produce a Convert or any similar tank. The bottom view shows that the tank's side walls wrap around where they meet he bottom panel. There's no way a single top piece could be stamped in this form; there's no way to wrap it around the male portion of a die or to remove it from the tooling if it were formed in that way. As I noted, it seems that there is a left piece, a right piece and a bottom panel.
