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The caulking mentioned is fine to seals the gape but you may need somethingto back it other thatn backer rod. in that case you may need a small amount of Rockwool insulation to pack into the void befor applying the caulk. This is a link to the product https://www.rockwool.com/applications/exterior-walls/firestopping/
If you only need a small amount check with a local insulation company to see if they will sell or even give you the smaill quanity you need.
Nice space. Its hard to tell from the photos just how big the space needing to be filled is however
Dap 18854 High Heat Mortar Raw Building Material, Black
should work well. It can likey be found at you local hardware store of online
Best of luck and Happy Holidays
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