I'm not going to doubt that. If Chrysler had extra go wings sitting around, they were going to use them. Most people wouldn't know the difference anyway. Chrysler's job was to build cars, and get them out the door, $.
But the general rule of thumb is:
70- Go Wing
71-on Gull Wing
On a side note, there was 2 J81 code options:
1970 J81 "Rear Spoiler" (Go Wing).
1971 J81 "Aerodynamic Rear Spoiler" (Gull Wing).
