To be precise, the 302 dated 7/31/79 states:
"advised that there is a ninety-nine per cent chance that this decal came from the same aircraft hijacked by D.B. Cooper."
When I first read this my understanding was that it meant the "decal" literally came from the same jet. That said, it probably means that it came from the same aircraft to the extent that it came from a 727.
Also, the fact that the decal was colored maroon and not bright red, as noted by NWO, appears to indicate that this decal is not from 305. Rather, it appears to be from another 727 of a different airline. What a remarkable coincidence if that is the case.
Finally, even though we know nothing about the decal, it is not likely that the deal is an airstair placard such as the one attributed to 305.