This lovely scan shows wing-margins on the left and right hand side of the 8th pane in a sheet of 12 panes,
each pane consists of 20 stamps in 5 rows of 4 stamps (240 stamps per sheet).

Scan courtesy Elstree Stamps

As singles you would normally see wing-margins like this

Layout of the pane
stamps in columns D and E, and H and I will have wing-margins

Part of pane also showing current number below stamp TK

Image source British Postal Museum from the R M Phillips Collection. Vol XXXIII Pg 56.

Wing-margin stamps occur on the surface-printed stamps 1855-80. For the 3d, 6d, 9d, 10d, 1s or 2s stamps with watermark emblem or spray the sheet is arranged in 12 panes of twenty stamps forming a sheet of 240 stamps, stamps with D, E, H or I in the SE corner will have a wing-margin. That is 80 stamps out of 240 or one third will have wing-margins. With the introduction of the Imperial Crown watermark in 1880 (SG 157) there are no wing-margins.

For the 4d and 8d garter series the plate is in 4 panes (2x2) of 60 stamps (10 rows of six) and thus the 6th and 7th letters, F and G, would be wing-margins, that is 40 stamps out of a sheet of 240.

The 1875 2½d stamp (plates 1-20) had two panes of 96 stamps one above the other and no wing-margins.

With the imperial crown watermark introduced in 1880 there were two panes of 120 stamps (ten rows by twelve) and therefore no wing-margins.