This is very interesting.
DionR you may be onto something with it being a production plant discrepancy...
My 1974 Plymouth Duster 360 4-speed (a good photo of the fender tag is posted in this very thread) was built on December 7th 1973 ('C07') with the bracket style front bumper attachments. It was built in Hamtramck, MI ('B' code production plant)
My other 1974 Plymouth Duster (VL29C4G) was built on July 15th, 1974 ('715') at the St. Louis, MO production plant ('G' code) also has the bracket style bumper attaching.
1974 Dodge Dart Sport #1 (LL29G4G) was built on December 14th, 1973 in the St. Louis, MO production plant ('G' code). It has the bracket style bumper attachments.
1974 Dodge Dart Sport #2 (I just purchased this past week from a FABO member - LL29C4B) was built on April 10th, 1974 (410) in the Hamtramck, MI production plant ('B' code). It has the bracket style bumper attachments.
I find it extremely odd that I have early 1974 model (late 1973 production date) and late 1974 model (early 1974 production date) cars and they all have the bracket style bumper attachments.
it's very strange that DionR's Duster with a November 21st, 1973 build date from the St. Louis, MO production plant could have the shock absorber bumper attachments and yet my Dart Sport #1 from the same St. Louis, MO production plant built a month later still had the bracket style bumper attachments.
In fact my slant six Duster built 8 months after DionR's Duster was still had the bracket style bumper attachments.