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I agree... to my expert knowledge, and I will testify for you... all 1969 had sidepipes. Especially those that aren't legal unless they were OEM...
Seriously... there are a few places to check on the car... but NOTHING to prove except the build sheet. You can buy one online, spill some coffee on it... wrinkle it... then show it to DOT.
Look in the rear storage area written on the inside ceiling you would see N-14 hand written. Look between the frame rails and the body in the area under the doors and you will see some insulation if it was a N-14.
If you want to see all of your options, you can peel back the carpet behind the drivers seat and you will also see handwritten versions of the options on your car. Mine had PW (power windows), PB (power brakes), and N-14 (sidepipes)