Help with 1975 Duster ID

Welcome aboard and nice looking Duster.
The tail panel is definitely 75. The reason that some of the decoders are sketchy is they stop at 74. Some of the codes may carry over some may not...

A lookup on the VIN will say it's a vailant. A Duster after all IS a 2 dr valiant coupe. The fender tag will show the color and "packaged" options, BUT if you want to know what it really came with, look for a build sheet under the back seat or behind the glove box liner.

Some sites that might help:
http://www.mymopar.com/index.php?pid=17
http://www.stockmopar.com/mopar-fender-tag-decoder.php

If it IS a "Duster custom" it originally had "custom" fender badges just under the Duster badge. It should also have the wood grain dash and a locking glove box and a AM/FM stereo. It may also have had wheel opening molding ( check the inside edge of the fenders for holes ) in addition to the rocker molding. Of course this is just from memory of what my duster custom