According to Galen Govier's white books, there were 248,243 total Dusters built in 1973.
214,043 for the U.S. market
14,341 198 slant 6 (VL29B)
153,255 225 slant 6 (VL39C)
46,477 318 2bbl (VL29G)
11,841 340 4 bbl (VS29H)
So i guess your car is almost as rare than a '73 340 Duster, but i bet it's not as desireable by a long shot.
This is one case where rare and desireable don't go hand in hand........