Cob Of 8 Reales, Minted In Potosi In 1730 (Essayer M)

Here is a beautifull cob of 8 reales, minted in Potosi (Bolivia) in 1730 by essayer M.
The 3 legal dates are visible, one complete and two partial.
It looks like the die broke during the strike, and the trike was redone just after.
The coin was made on a very fat and crude silver piece, it's beautifull :)

I wonder what happened to the grid side - the reason for this break (the dies were done that way ? or it's an over-essayer crudely done ? it's very odd ...)