That Blowhole button is as nice as any I have seen posted here. Congrats on finding what looks to be a great site. Beautiful buckle and I like the other buttons too!
Sorry for not giving some explanation earlier on. The big button with the two holes drilled on the back is the blow hole button, the one made from two pieces. It is called a blowhole because of the holes that were drilled on the back, they were used as vents for gases that were produced when the button was made. These buttons date from the late 1600's to the mid 1700's making them an early find.