My guess would be a junkyard 'upgrade' in the first 7 years of its life. Typical back in the day mod to get a deeper set of gears. Not a factory install. Seven years is a long time, guys modified their cars right away with over the counter parts. J.C.Penny stores had a performance parts department back in the day, anybody remember A/FX slicks in the store?
I agree, it was "removed". In the 70s I personally removed three Danas from GTXs, one being a 69 b-5 blue convertible GTX (bought for $125) used engine trans and rear in another car. Scraped all three. Chevy guys wanted those Danas bad for their race cars, even used the four speeds. At the time the 70 was like a ugly sister anyway so no big loss. No such thing as numbers matching-restoring mindset then, didn't have a clue what we had, and all dirt cheap.
