I would guess it is not original (69 was 1st yr. of W) to the car, from what I remember on my '70 351W block the D0XXX number is stamped onto the left (drivers) lower block flat ridge to the rear of the motor mount, just above the oil pan bolts. If you can jack the front corner of the car up and slide under with a good flashlight you should be able to see it, may need a rag for grease on it. The letter is the decade C=60's, D=70's, E=80's then the next # is the year, D5XXX = 75, etc. Jon