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Pass X - Simply means Passenger service but doesn't mean the heads where not used on trucks.

336781.....73-85...oval...OPEN...454, 118cc chamber, 256/114cc ports.

K21-77 - Is the date: Nov 21st 1977

12T - I believe is the Mold # used for for that pour.

Popular Large Oval Port Performance Head for 427 and 454 MK IV's with Static Compression between 9 to 11:1 (Pending Piston Dome Volume) and good for 6500+rpm on pump gas. I use this heads with 2.19/1.9 Valves, minor Bowl and Runner Porting on a 10.25:1 7000rpm SR 454 MK IV.

About the only draw back is the Weight of Iron Heads make the front end Heavy and affects handling.
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