I am with Hot Water - I would have guessed the drop-bottoms. However, since then you have made it clear.
I think the definitive mill gon was the Atlas plastic - K-Line duplicated it in metal, but cheaped out on the underframe. Had they not done that, the K-Line would be the best for those of us who do not model in plastic.
The drop-bottoms can be fixed with brass center sill extensions quite easily, and make for an inexpensive way for SP fans to have appropriate gons.