No, K & N's are not stock, but they might as well be since that's the first thing any of us does! Sounds like the previous owner put it in for you. What self-respecting Mustang owner doesn't put in a K & N?
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"Back in the day"-Red '66 289
"Later"- Black '68 289
"Today"- White 2000 GT
Stock except K&N & Flowmasters
ET-13.952@98.87 RT-.507 60'-2.09
Son #1 - '87 GT stock
Son #2 - '89 LX 5.0 stock