1978 Ferrari 308 GT4 front bank not firing

The yellow wires (terminal 15 on the coils) are the +12V signal from the ignition switch -- they should both be at +12V (relative to ground) whenever the key is "on".

No and yes. There are no cranks sensors, but there are two sensors inside the distributor --- each one fires its corresponding coil. Items 36 here:

https://www.ferrariparts.co.uk/diagram/ferrari/308-gtb-gts-carburetor/042-engine-ignition-(from-car-no-23561-gtb-and-23265-gts)

You could swap those, and see if the no spark condition moves to the other bank, but you do need to set the proper spacing to the reluctor wheel on the distributor shaft -- see page 12 in your 155/78 OM "gap between stator and rotor (distributor with impulses)".

 

I would -- too fiddly to try to do it with it mounted on the engine. Just make a note/mark for where the dist rotor is pointing (and put it back in with the rotor in that same position), and make a mark across the dist housing rim and the camcover flange so the dist housing can be put back in the same position. JMO.

Did you already confirm that the wires on terminal 15 of the coils are both +12V (relative to ground) with the key "on"?

 

That (now-exposed) wire stranding is the ground shield so not a huge deal that it has lost its insulation in that area (as its loop clamp connects directly to ground with the mounting screw so if those strands are making contact with the metal dist housing that's what they do anyway). The two small insulated wires are the "impulse sensor wires". Of course, the corrosion showing on those exposed shield strands is not a good thing.

Can you describe what "fiddled" means? Does that mean you removed the distributor, swapped the two impulse sensors, swapped their connectors at the coils, and got everything back together as it was?