I has been said the Spanish name for the coins was “cabo de barra”or end of the bar because it was cut off the end of a bar of warm silver and then hand hammered with a die on an anvil and the other in hand. The English dropped the o and pronounced it cob. The coin was then cut down to the proper weight. That’s why many of them have one or more straight sides.
If you have questions about cobs The Practical Book of Cobs is a great resource if you can find one. Keep swingin.