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IIRC the only real diff is the 12 uses a tube focus rectifier and I think octal based sweep tubes. Maybe a different video out as well. the one thing for sure is it does NOT have the blanker tube bias connection to the horz out tube grid. the 15 and forward had this as a protection from a shunt tube failure. If the HV rose too high the rectified pulse from the fly at the grid of the blanker is tapped with a voltage divider and fed back the the horz out grid. as the HV skyrockets with no shut tube load, the increase neg a the blanker bias off the horz out tube.
Something to look for if you ever have low HV and higher than normal neg readings at the horz out control grid. this presents as poor HV regulation and maybe poor width. there is a voltage divider resistor that connects to the ground thru a thin PC trace along the edge of the chroma board that is right next to the horz out tube. If that trace crack and opens (high heat from the HOT and flexing of the board to replace the 6GU7 blanker tube) the the voltage divider is upset and you will get a large fixed neg bias at the horz out tube grid that is NOT a result of the horz osc drive.
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