Yeah ... I'm really leaning swirl.
Basically, Dan, the swirls are mostly West Virginia marbles made after 1930. There were around 10 companies making them. I can't remember the exact count.
The slags were an earlier style which started with M.F. Christensen around 1903, and were picked up by a few other companies -- Akro, Christensen Agate, Peliter -- in the 1910's and 20's -- and then were mostly phased out around 1930.
So, when we have a transparent-based swirl, we usually call it "Transparent Swirl" and leave it at that. It's almost impossible to nail them down to a particular company if you don't have original packaging or didn't dig them up on site.