Epilogue: monitor fixed! We purchased a NOS CPT1516 flyback from Ceronix, which was a Meritron 8090B.
My friend says:
Some of the Ceronix 1493 VGA models were shipped with a flyback that did not have an internal capacitor—which is what the extra pin is connected to. On these units, resistor 465 (480ohm 1/2W) was not needed and also not installed. I missed this originally as there is solder in the attach points from the original reflow, but no component was installed when looking from the other side.
Installing a 10-cent resistor from a surplus store solved the problem and the monitor looks great now.
Here's what it looks like (yes, I bought another game while awaiting it getting fixed):
https://youtu.be/rZwasqK63hM