All Mustangs in 71 & 72 came with chrome front bumpers as standard equipment, while Mach 1s came with the eurathane front bumper. As mentioned above, there were a couple of optional trim packages in 71/72 that included the Mach 1 style eurathane bumper also.
For 73, all cars sold in the US had to have 5mph front bumpers and 2.5mph rear bumpers, so the Mustang's bumpers for 73 reflected this change.
The new 73 eurathane front bumper was then included on all models...no chrome front bumpers on any 73 Mustangs.
The rear bumpers on 73s are the same as 71/72, yet are mounted an inch or so farther away from the body with some rubber spacers to hide the gaps.
Many people are mistaken about what a "5 mph bumper" is supposed to be designed to do...they think it should withstand an impact up to 5mph with no damage: not true.
The regulation was that a direct frontal impact of up to 5 mph could not damage any critical or functional part of the car...cooling, steering, engine, etc...
There is no regulation as to cosmetic damage.
I remember when the 73s were new, the new HUGE front bumpers combined with the 14" wheels/tires on the Mach 1 made the car look badly proportioned to me, I hated them.
But they grew on me and now they look just fine to me.
I had the same reaction to the new 79 Trans Am front end when it was new...I thought it was horrible.
But it grew on me...I love them now.