Q:

What's this black rectangle painted on the side of a V-1 flying bomb?

A friend sent me a photo of the V-1 flying bomb on display at the Air Zoo in Kalamazoo. What's the black painted rectangle on the side of the fuselage?

The writing isn't much help... Nicht auftreten is the German equivalent of "No step," but I don't know what Abstellpallung means, except that abstellen is "to cut off / shut down." I can't find Pallung in any German dictionary. The label appears to be pointing to the black rectangle, which appears to just be painted on.
I also can't find this rectangle in online V-1 photos.

A:

A "Pallung" is a piece of wood (or something similar) that prevents ships (or something similar) from falling over when they are on land. "abstellen" means to put something down (such as luggage) or into storage. There are some recent articles that contain the word "Pallung" on google news, such as this one.
Assuming that the book depicted in this pdf is authentic, you can see what an Abstellpallung for a V1 looks like on page 29, indicated by the letters b and c:

Note that the text twice says "Markierung am Rumpf beachten!" (mind the markings on the fuselage), which probably corresponds to the black rectangles mentioned by the OP.

(Some native German users have indicated that they have never heard the word. Neither have I, until today. I am not sure about other languages, but in German, some domains have very specific vocabulary that a layman will almost never get in contact with. One example of this is anything connected with ships, another one is hunting, and there are probably some more. The manual contains another example just in the preceding sections (what does "heißen" mean here?). So "I have never heard the word" may just mean that someone is not familiar with some particular domain.)

A:

abstellen is "to cut off / shut down."

That's one meaning of the word, what we need here is "putting down", "parking" or "putting in storage".

I don't know what Abstellpallung means. I can't find Pallung in any German dictionary.

Me neither. It seems to be a quite rare nautical term, meaning "strutting", "bracing" or "propping"1. A "Palle" is a (wooden) support block on which ships would rest in a dry dock.
The closest match I found in an actual dictionary is the adjective "pall" from the Low German language [1] [2] which means "steif" or "fest" (stiff/firm) in German, and certainly could also be used as a verb ("make firm", to strut). Even the Duden knows the noun "Pall" from the same word origin, as nautical term for a pawl (ratchet) that fastens a capstan.

What's the black painted rectangle on the side of the fuselage?

It appears to be the point where the fuselage is meant to rest on a support frame for storage. I would guess that location is reinforced by the internal structure2, like @PeterKämpf said, for jacking it up.
1: Not sure what the subtle differences between these terms are, I'm not a native English speaker.
2: Or not.

A:

Contrary to some other answers Palle is actually a real German word, though it is used mostly in context of shipyards. A Palle is a wooden or concrete block on which ships are resting in drydock.
See also this German Wikipedia entry:
https://de.wikipedia.org/wiki/Palle
A 'Abstellpallung' thus means a location one the V-1 where one can safely support it on a Palle for storage or maintenance. The black rectangle marks that location.

