FINALLY GOT IT BACK!

That could be a factor. Late 71's may not have had them. Every car I had that had the stripe still on it did. I have old ads from the 70's showing the 340 on the back on 71's . Also just the Duster and dust storm on twister's and standard models.

I never had a 71 Duster 340 with a blue engine or slotted bumpers like yours. I know they had them but non ever went through my hands. Many 72's though. Every 340 Duster I ever had had the Duster 340 decal.