There are 4-1/2 different types.
First were looped at the top (Super rare now), but these were reworked into wings (quite rare enough).
Then wings.
Wings modded and modded holes in side, during the Great War.
Straight-up type, used only in India, mostly post-Independence. Not on my 1943 Ishapore, on my 1948, so I don't know precisely when the change-over came.