9 1/4 rear end

last summer i bought a '74 duster, after checking it out after the buy i noticed it has a 9 1/4 rear end. Im curious if this rear end was an option or if it may have just swapped in sometime over the years.

Does it actually have the 9.25 installed? I didnt think any were even close to short enough to fit under an Abody without modification.