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.
