1966 Kennedy Half Dollar SMS Ridges Separated By Horizontal Line

Ok ,lets look at this .
The planchet was punched from a coin metal sheet. This process will actually leave a slight protruding ridge in the middle of the then blanks edge. This ridge is one of the things to look for with a curved clip. So far there is no groove around the coin.
Next , the upsetting mill rolls the blank to give it a protorim and the exact diameter for feeding into the striikinng chamber.when this is done the blank becomes a planchet. For the groove to happen at this point , a upstanding ridge would have to be present on the wheels of the upsetting machine. Not something that I have heard of.
The next step is the striking chamber where if this anomaly were to form there , a ridge would have to be present on the collar, though if that were the case the finished coin would never get ejected from the collar.
This is why I said that I didn't know how it could have happened until after the strike.

It appears to have a DARK line in the center of the edge, ut in the last image that dark line does not appear to be either higher or lower than the outer layers.

It looks like toning at the center of the edge/reeding. I'm checking my sets as soon as I can to see if they have toning or discolored areas.

Here's a comparison of two 40% clad coins.
Maybe this will help. The one on the left has the odd edge.

In the last picture it is clear that the center of the ridges are not depressed which they would be if it was actually a groove. It is some kind of odd toning.

And a steady hand...