I had a 1974 455 YX code 455, it had 4X-1H heads on it. The 1H appears on a squared off cast-in hump on the exhaust side of the head near the valve cover land area. There should be 2 of these cast in humps on each head, approx. located between the center and end exhaust ports on each side. Look at the head so you are looking at the exhaust port exits, than look at the fase of the squared off humps I described above, on one of them you should see a little punch type stamp of a number and letter.
Like I said mine was 1H
Hope this helps,
Mark
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1965 GTO: 467 (Built By Dan Willever), 400 Block, Eagle 4.25 Stroke, Eagle 6.8 Rods, BRC Pistons, Custom Grind Hydraulic Roller Cam, Edelbrock 72cc Heads Ported, HS Rockers, Doug's Headers, Edelbrock Victor 4150, QTF 850 Carb, AutoGear M22 4 Speed, SPEC Clutch and Pressure Plate, 12 Bolt 3:73 Posi.
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