Welcome to the forums Steve
In my opinion the 6d has been machined sometime AFTER issue from the Mint. The difference in weight and thickness shows it's not a regular planchet, and a trial such as you suppose would be the same width and weight as a regular specimen.
It would be good to see pictures of the obverse and reverse of the 1860 fathing. It might indeed be a toothed/beaded mule but do be aware that the toothed border can and often is, mistaken for beaded.