1789 Potosí 8 Reales Charles III Authenticity?

Potosí coins are all over the map in terms of quality. I agree with you that theoretically the punches should mean the lions are the same, but there are a lot of elements on this coin that should be the same but are not. You can look at the edge overlaps for more information, unless it's not your coin. I haven't heard of a good way to visually determine authenticity of coins of this mint other than having them in hand and checking the specific gravity.

The die sinking on the reverse is much less precise than seen on issues from just a few year earlier. I see some die doubling on the lower lion that may account for the different appearance of the tail. The low weight on this coin without circulation wear is suspicious as it is below tolerance and should have been caught in assay.

However, I see many punch characteristics that match Potosi issues of this period. You should have no problem getting it authenticated by one of the TPGs (although, the effort they put into authentication is suspect).

Here are my three examples from this period: