Yes, the eyes on top of the housing are in different locations and the angle of the eyes is different.
I agree, because I have BOTH body styles (76 Cutlass, 68-70 Chevelles), and I have discovered something that few people are aware of.
For the 73-77 A-bodies there WAS 12bolt rear available. I have found a VERY, VERY few of these 73-77 style 12bolt rears on SOME, repeat, SOME El Caminos and Cutlass wagons------------and maybe some Pontiacs(?????).
The down side of these mid-70s 12bot rears is that the gear ratios were VERY LIMITED, such as 2.73-3.08!!! AND, AND, AND they were available with an Eaton posi. I have one of these 12bolts (from a 76 Cutlass wagon). The 76 12bolt that I have is a 2.73 NON-posi, BUUUUUUUUUUT, one of my boys came across a Pont 2-series posi unit that I plan to install into the 76 12bolt rear.
From what little information that I have acquired about these mid-70s 12bolts is that the 73-76 versions had 31spline axles/axle gears and the 77 versions had 28splines.
These mid-70s rears are often over looked BECAUSE the differential cover has only 10 bolt holes and 2 big notches, thus, it seems most people think they are a 10bolt rear. NOT!
Oh ya, and the mid-70s Eaton posi unit is bigger and more robust than the 65-72 style Eaton posi units.
Here is what the cover of one of these mid-70s 12bolt rears looks like. This type of rearend has the typical fill plug on the side of the cast iron case, BUUUUUUUUUUUT, I discovered that SOME of these rears have a cover with a fill plug as below. In my book, that's a plus.
Here is a comparison of a 65-72 style 2-series (NON-posi) 12bolt differential and a mid-70s 12bolt Eaton 2-series posi (right) differential. As can be seen, the mid-70s posi is noticeably bigger than the 65-72 style. Also, the mid-70s ring gears used larger, LEFT hand thread bolts.
Soooooooooooo, if you should locate one of these mid-70s 12bolt posi rears, they would be an excellent rear to have------------------if you can live with the limited, much higher gear ratios.