Possibly Soundstream's very first run of the Reference 500 / 6.0 amplifiers before switching to the more familiar and standard silkscreen on the back cover.
According to Wade Stewart, he is confident this was a legit silkscreen used back when the models were first released. Unable to confirm if these were actual prototype revisions of the Reference 500 and 6.0.
Above is a pair of unfinished Reference 1000s amps that were factory built as "cheater" amps for a Soundstream engineer. The only difference is the etching on the plexi cover says 700s, not 1000s. These particular amps were rebuilt and modified to do more power than rated; without sacrificing the legendary sound quality that old school Soundstream amplifiers are known for.