Another 727 trans id question

I was looking through several transmissions I found in my garage and am curious about this one. I read the numbers on the pass side as PK3515843 3540 1997. I believe the build date to decipher as April 7, 1971 but I'm not sure what it came from. Is this correct? Also, what does the 1997 mean?
Thx folks!!
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