On my '74 Duster 360, power disk brakes were standard. Looks like that isn't the case with a '71 Duster 340 though. But based on your earlier pictures, the booster was there and I highly doubt someone went to the effort to add it. Also, if it has disk brakes (can't tell from your pictures), makes sense they would be power.
Might be of interest to look at some of this info, it's where I go look for things like standard options and such.
The 1970 Hamtramck Registry Library Page (1970 - 1974)
In regards to options and such, the info is under Dealership Data Books and then Valine/Duster
The 1970 Hamtramck Registry - 1971 Plymouth Dealership Data Book - Valiant/Duster
