On the product description, the compatibility is stated at A6000, my model is the A6003. Will the back casing be the same for my phone? 
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There are two models of the Oneplus 6 - A6000 which is for the Asian markets and A6003 which is for US and EU. A6003 has 3 extra network bands namely 30, 32 and 71.

Different LTE bands. The A6003 has more LTE bands, but it's more of a concern if you're in the USA and using T-Mobile, IIRC.
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