AC flyback has a connection that bridges the primary to the secondary. Why?

[extremeodd]

I'm needing a quick sanity check here. I aquired a couple of vintage NOS disk type ac flyback transformers off ebay a couple of months ago, coated them in several layers of super corona dope to repair the damaged insulation and put them up. Today when I went to hook one of them up to an expiremental ozone generator, I noticed both of them have a lead that connects the outer edge of the primary coil to the start of the secondary. I'm assumed this would be considered the ground lead (the only explaination I can think of) since one of the two flybacks had 2 HV leads red/black with the black one hooked up to that lead.

However, while I was testing the other flyback it didn't seem to want to work when using that lead but would work great using a connection that appears to be connected near the start of the primary. Sadly due to my disorganization I've misplaced the boxes that had the factory diagram and if it isn't obvious I'm quite the novice when it comes to high voltage equipment. I'll post some pictures here in a bit, had an incident when trying the "Find the flyback's ground by seeing which connection the HV lead has the strongest arc to" and had the arc jump from the connector to the wire which almost instantly melted down into the primary windings ( it wasn't the secondary) causing some damage. I brushed a ton of super corona dope onto it and now I'm waiting for it to dry, once I can touch it I'll get some pictures since I'm probably using all the wrong terms.