Answer

Mexican-shell stones manufacturing started in full swing in 1965, when our firm developed a cutting method using a diamond drill. In 1975, I built Mexican shell cutting factory in BAJA California, South State of Mexico. Since then I visited there 8 times and trained local staff on how to select good shell and cutting method.
However, we were unprofitable due to the decline of demand, we closed down our own factory in 2000. Since then we purchased Mexican shell, which is already cut out at our affiliated firm.
Distinguishing Hyuga and Mexican clamshell is relatively easy because these two have totally different grain patterns.