75 dart sport front bumper interchange

The Valiant, Sc amp and Duster models were all the same for each year since all models used the same front end sheet metal.
The 1974 is another sort of unique year because it was the changeover year for the 5 mph impact redesign.
Here is where I am still unclear.
I used to think that the first few months of the 74 model year, that they came with regular fixed mount bumpers and that the 5 mph bumpers with the shock absorbers were integrated later. I've found that to be wrong. I've seen and/or owned early build cars and late build cars and seen both designs in several models. In short, it may have been a matter of what a different assembly plant had on hand to build the cars.
I assumed that it was like it was with other things. The Plymouth Superbird had to be be built before January 1970 to comply with new bumper regulations that went into effect January 1st.