Some of the 1 inch Pace shims are longer than the fork crown, so they stick out of the top of the fork crown and are tapered for the headset crown race to press down on.
mikeeb The one you describe is the reducer for a 1"1/8" crown to 1" steerer the reducer is machined to fit the 1" crown race. Pace detachable steerers are supplied as parallel tubes hence the bottom is not over sized for the crown race to sit on that's why you require the crown shim
Retroking you are right. I assumed this was the set up being spoken about as all the Pace forks crowns I have owned (RC30/35/36/proclass) I have owned have been machined for 1" 1/8th steerer and thus have all needed the reducer shim for 1". If there were crowns produced machined to 1" I was unaware.