For the 4d plates 1 to 17 and the 8d (garter wmk) there were four panes of 60 stamps arranged as below. The wing-margin stamps would therefore have either an F or G in the SE corner. That is 40 out of 240 stamps in the sheet will have wing-margins. The plate layout for watermark Imperial Crown in 1880 is different and there are no wing-margins.

Current number is above AB and below TK (or above AA and below TL)
Plate number is above AK or below TB (or above AL and below TA) early 1855 4d had plate number in all four corners.