A brockage error can only occur when there are two coins involved. One of the coins involved will always be a struck coin which has not ejected properly. That struck coin will find its way back between the dies and will be struck next to a blank planchet which was fed into the collar. The image of that first struck coin will be impressed into that side of the blank planchet. The result will be a second coin which has images of the first coin impressed into it. Those images will be pressed into the coin and the image will be in reverse. This incuse sunken image is known as a brockage.
This Type 3 $1 Gold brockage is unique for the type, and only 1 of 2 known in the entire $1 gold piece series from 1849 through 1889. The other known piece is a Type 2 $1 Gold brockage and was authenticated and certified AU 55 by PCGS. It is a first strike obverse brockage of the reverse. It is in choice mint state condition with proof-like surfaces. This spectacular coin is from my personal collection and has been authenticated and certified by NGC and BYERS COLLECTION is noted on the insert.