Listed in Scott as 540a, type III vertical pair, imperf horizontally. This and Scott 538a were issued because of a misunderstanding about requests by collectors for imperf top margin coil waste examples. At some point in time, the Bureau had switched from imperforate top margins to imperforate bottom margins when perforating coil waste using flat plate perforators that had ten perforating wheels.
It is a long story well documented in the philatelic press.
See Kelleher lots 2936 to 2944:
https://stampauctionnetwork.com/V/v71197.cfm