I have had a quick look at a couple of the net sources and I found that the fabric patch was in useage in Burma, China and Aleutian islands mostly. The protection of the slot was unpainted very often. But it seems that its colour depened on the fabric one rather than the painting by the maintenace crews. In the Kagero book for the P-40 vol.2 there is an image of a P-40K of the 16th FS, the 23rd FG in China with the unpainted, light ( white? ) fabric protection of the slot noticed nicely. Below what I found ...
View attachment 805075
View attachment 805079
View attachment 805076
But using of the patch wasn't a rule ...
View attachment 805080
Contrary to that the P-40s in the Africa and possible the Europe didn't have it covered at all ...
View attachment 805078
View attachment 805077
the pic source: the net.