8:9a There was nothing in the ark except the two stone tablets that Moses had placed in it at Horeb,

The box only held/contained the two flat stones that Moses had put there on ⌊Mount⌋ Horeb.
-or-
The chest was empty apart from the two flat stones ⌊with the Ten Commandments⌋. Moses had put them in the chest when he was on Horeb/Sinai ⌊mountain⌋.

8:9b where the LORD had made a covenant with the Israelites after they had come out of the land of Egypt.

There Yahweh had made a covenant with the Israelites after they left Egypt.
-or-
After the people of Israel had left Egypt, Yahweh had made a solemn agreement with them there at Horeb/Sinai.

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