Picked this up at a local bike show. The owner says it's a prototype/preproduction 1989 Schwinn flatland pro frame. I did a comparison myself and confirmed it had many differences from a production frame. This frame has no holes in the rear brake mount. The rear dropouts have a notches on top and they are smaller. The rear coaster brake mount is spot welded on. The serial number starts with GP. This is a NOS frame. The dropouts have no markings from a wheel. Here's some photos.
I'm skeptical of any claims of a prototype on anything unless there is research and documentation to back it up. You have a nice a find. I would just get this checked out by multiple Schwinn and BMX experts and historians and have any confirmations that is a prototype done in handwriting and signed.