Talked with Dick Teal earlier tonight. Very nice guy to chat with. He believes the sled IS a prototype/pre-production sled for the 1978 model year. It should have been destroyed but somehow sneaked out.
Looked through a 1978 operators manual after talking to him. The pre-production sled pictured also has the serial number tag in the 1977 location with the liquifire decal covering it along with the gusset cut short to clear decal. The tag hidden on the bulkhead and not on the tunnel could have been due to photo shoots. The serial number on the tunnel of this sled was never registered with Deere.
The pre-production suspension pictured in the operator manual is also the same one found under this sled. (The dator lite suspension pics I've seen have not had the gusset under the front hinge.) Also in the pic, notice the dimples punched into the floorboard, but no hole through the middle of them can be seen-same as this sled. '76/'77 tunnel floorboards were smooth and production '78s had the punched dimples with holes through them in the same location and pattern as this sled.
Any chance of finding one of those big hood vents to put back in the hood?
This thing is definitely worth restoring!