Ok. I have an idea for how we would use a Hill cypher for the post it notes. I could be very off base, but I've been doing my best on this and I have some convincing evidence.

When you look at the grid, I noticed a few things. For this I will be referring to the post its by what is contained in them. I'll first start with the ones with enough concrete evidence to make me believe it.

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• Butters = FNAF 6: sister location. This one seems to be the obvious one due to the nature of of what is presented at the end of the game. This means that the top left number would be 6. Now the reason I'm saying 6, is because for this I included the spin-off games as it is possible they do, in fact, play a part for this cypher.
• Scribble = Mangled's first appearance which was in FNAF 2. While not completely identical to the scribble, it makes sense to have it represent mangled since Sister location's number has already been used. Following this logic, it would also be fair to assume it is mangled, because she was also the very first one in the series to have anything like this concept mention. this means that the top right number is 2.
• Footprints = Shadow Freddy. This is the weirdest one I know, but after looking at different models, something began to stick out. Feet models. (Don't look it up, it isn't worth it promise, but I did.) I noticed that through all the different Freddy renditions, they have flat feet, with the except of shadow Freddy. This also plays into the fact that they finally added curvature to the feet. This in conjunction with the idea that shadow Freddy would be related to the midnight motorist footprints, due to the speculation that he would have been the only one who could leave the pizza plex without malfunctioning, only makes this more convincing. Shadow freddy was an easter egg in FNAF 2, but was a secondary antagonist in FNAF 3, meaning middle left is the number 3.
• Hill and Sun = Not sister location mention. Think about it, there was more than one hill in that image everyone has been connection it to from Sister location. That being said, it wouldn't make sense for the context of the picture. So looking through many pictures there is one other game that has a sun with a hill. Security Breach. The True Ending has the mentioned image, with the three of them on the hill, looking out with the sun in the corner. While it is not a perfect idea, it would make sense. This means that this number would be for Security Breach, which in the technical timeline of game releases, would be game 17. That means the bottom left would be the number 17. [If yo udo not include the fanverse games, this number could be 14]
• Star and Mic = Lefty from Pizza Simulator. It cannot be something that represents Freddy because of where the microphone is. Lefty, as his name states, is left handed. This means when we look at him, we see the microphone on the right side. [refer to the image attached for my idea, as I don't think I can explain it well enough.] This is also confirmed, to me at least, because Freddy is right handed, meaning the microphone would have had to been on the opposite side. This means that bottom center should be the number 7, since Pizza simulator was the 7th game released.
• Candy = Candy cadet, but not in Pizza simulator. The Candy Cadet makes another appearance in the series within the RUIN DLC. What is so funny about this, is many people would automatically go to the 7th game within the list of games from the series. I cannot because it also wouldn't be 18 as it was not the next technical game to come out after Security Breach. Ruin, is classified as the 20th game, included direct spin-offs and Additional games Scottgames helped publish. Now this number could change, if you do not include the fanverse games. [for the sake of some people, I'll give you the number without the fanverse games within brackets. This means that without the fanverse games this number would be 17]

Now with all this said, I am still attempting to decipher the last three numbers. These I do not have complete proof of but I can give you my thinking for the final three.

• Hearts and Pizza = All I can think is this has to do with Chica in some way, shape, or form. For whatever reason I believe it would be one of these two numbers. Either 5 for FNAF World. While she does attack with pizza, I am unsure of how the hearts would be incorporated. My better guess, however would be the game directly after Ruin, which is called Freddy in Space 3: Chica in Space. This means the number would be 21 [or 18]. My reason for believing it is the later, is because it involves Chica who has a floating pizza slice who follows her around, and there are literal heart trees in the environment. This gives me a true connection of this being the answer. So I will be putting those two numbers in the center of the grid.
• Three Lightning Bolts = This one has to involve Glamrock Freddy, but believe it or not, Security breach is not the first game he appeared in with the lightning bolt. He actually was in Security Breach: Fury's Rage. This is important to note that this was released before the main game, due to the delays. This game, just like Chica in space, are fillers or games made for GameJolt. This means that if this hunch is correct, the center right would be number 15 [13]
• 7 Dots = This one, stumped me. I looked for a while, and couldn't figure anything out until I looked at Special Delivery. Now, Special Delivery did not take off really well, so it would make sense this game would be largely ignored, but this was an AR experience that game something new to the table. Following Help Wanted, it makes sense that this COULD play a role in this grid. The 7 dots remind me of the remnant collection in the game. They would appear with the black versions which would hurt you if collected. Now, this is speculation, but I feel confident enough to say this could very well be the answer to this post it note. That means the final number is 11.

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This is my theory on what these post-it notes could mean, and I do believe it falls within the numbers allowed to me used in a hill cypher. I could be wrong, but I wanted to put this theory out here, as I have some good evidence for some. Let me know what you think.