Seam is the easiest thing to explain, and the reason is the same of what you think: resize a ring. What you write is wrong, since the mold was only 1, and it would be a waste of money to make tens of different molds for the different sizes, it is easier make 1 only and resize the ring. Skull is cast too. And costs are almost the same: 1 mold and nothing more (2 if we include the skull).
WRONG
I used to make cheap TK rings by casting for some wholesellers too. Since there was need for different sizes I had 10 "rubbers" (rubber molds) for diferent sizes. I spent one day making 10 diferent size models, one day mor making rubbers and saved weeks on labor to resize rings. Each rubber mold costs less than $10.
There are less original rings than you think, much less...
