Anytime I see stuff like this, I get curious. The two images to follow are seat back frames from a 1972 Challenger. The broadcast sheet inside was mouse fodder, so I do not have a scheduled production date for the car these came out of.

Image 1, note all the holes for mounting the spring 'basket' are there like 1971 model year seats had, but only the top rear spring was installed. The top rear spring was the only one mounted independently of the spring basket in other 1971 seat frames I have seen.

The date code on this frame is the 216th day of 1971.

Image 2, no spring, no holes to mount it & no holes for the 1971 style spring 'basket' either.

Date code on the frame tag is 124th day of 1972.

Sorry I don't have a picture at the moment, but after many years of taking apart E-bodies I just noticed a new one.
Later 70 model years have a bracket with 2 studs sticking through the firewall where the clutch/brake support bracket attaches. This bracket is only visible if you take off the Drivers Side cowl screen.
Because of these studs, the Clutch/Brake Support bracket has 2 oval shaped holes in it.

I'm sure others have noticed this, but I just saw it.

1969 B bodies...Mid year change in rear panel where gas tank filler neck goes through panel to gas tank. Changes from round to oval about January.

My June 1970 Barracuda the LH door window slide is 70 style the RH door has door glass slide thats the newer '71-4 style.

Also the rear brake hose was the '71 style setup.

Looking at the exterior of this early 70 ebody, at the bottom at the e-brake cable area, you see two large holes - one where cable enters. My car is the newer '71 style panel only has one hole for cable, and the firewall has one hole that usually don't see on other70 ebodys.

early 70 ebody firewall...

are there other late '70 ebodys that have this same hole - looks like you place a wire holder for voltage regulator in that hole?