What the rubber bumper is going on in 74

Ok, so I made a comment in a thread about a Mexican 74 duster having the oil can bumper mounts on the front bumper. Before that I was happy thinking that 74 had the conventional bolt to frame type and only 75+ had the new four the time oil can mount.

So I go take a pic of my 74 duster to show the OP of that thread the difference. I have 2 74 dusters in my yard and to my surprise, they are different.

Here is where there is no rhyme or reason .... maybe. The brown car with the older type mounts is a later sequential number than the blue car.

The brown car is VL29G4B44
The blue car is VL29C4G20

So the slant six car with the earlier sequential number has the latter style of mount

What was going on in 74?

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