Ju 87 Stuka markings

Hi all,
Does any of you have an guide to when and how the Stuka carried underwing letters? My collection of pictures and books doesn't give a clear idea.
Some have no letters under the wing, some have the full four caracters identical to those on their sides, some have only one letter (Aircraft individual letter I guess as the third often in different colour) and some even have what seems to be the stamm kennsigen from the factory and a different code on the fuselage.
I am building a Ju-87 C-0 preproduction plane in 1:32 and decal from the kit (Revell Ju 87 B-2) includes both the right Geswader codes and insignia for the carrier group that became a regular geswader when Graf Zeppelin was canseled.
My best gues is that it should have the third letter from the fuselage repeted in black under both wings.
Claus Gustafsen Strandby/Denmark
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