I had shared some thoughts privately with others who have encouraged me to post them here.
"The rings were cast round in a rubber mold"

If these were cast in the round what is the purpose of the seam? I've seen rings from size 7 to size 16(American measurment) so were there different size molds?* If you cut these round castings apart to resize them wouldn't the ring now be out of round? Why don't we see other types of cast ring examples with a seam if this was a common practice?
*Also if these were in fact cast round wouldn't you occasionally cast one which was the right size for the order where no resizing was necessary thus making cutting apart and a seam unnecessary? But we all know of no legitimate rings without the seam.
Just some more food for thought.
Jim