I had a couple that showed a similar "skirting" on them and they were Lee molds.
What I did that helped was to remove the sprue plate, and then take a good bench stone and using the smooth side I cleaned up the bottom side of the plate. I didn't get overly aggressive with it, but simply let the weight of the stone do the work.
I put a couple of pieces of that blue masking tape down on a piece of 1x4, and then laid the plat on top. The tape had enough friction to the outside that the plate was held in place really well. Then I added a drop or two of light machine oil and started doing the figure 8 on top. It immediately showed areas which were high, and I kept at it until most of the plate showed at least the light marks of where the stone rubbed. Like I said I only used the weight of the stone, and didn't put much if any pressure on it. I didn't want to take a chance of hitting one side or end more than the others.
When I was done with the plate, I cleaned the stone up, then did the same thing to the top of the blocks. Same thing, only using the weight of the stone to do the polishing.
When done I cleaned everything up using brake cleaner and reassembled did a couple of heat up and cool downs, and repeated spray with the brake cleaner, just to be sure I had the oil out of everything.
I relubed the pivot points and the plate up under the screw with some Bullplate, and the next time I had the pot up to temp I poured a few through them and no more tail fins.
Sometimes what I have seen with the Lee molds, especially the 6 cavity ones, is that the sprue plate is a bit concave, higher in the middle. I figure that this is from being stamped out rather than being cut, but I don't know for sure how they make them. I DO know that if you take one that is brand new, and use the stone on it, that it will clean up just about, if not, all the way around the edges, before you ever get to the center portion. That is what gives the the idea they have been stamped out.
Hope this helps.