From Zebra: "I can't find a $1, but the entire series can have the overprint horizontal, vertical, or diagonal."
I searched the PF certificate database, and they only have one.
None on ebay. Seems pretty hard to come by.
EDIT: Found a lot of them sold at Kelleher: https://stampauctionnetwork.com/V/v779103.cfm
rod222: I updated the thread title to be a little more specific