1699 William III Sixpence Opinion

I do NOT have many examples of this coin - I don't even have one from each mint but I have maybe 30% of the extensive varieties. All the sixpences of that date (and the few years before and after) have huge varietal and other die differences.

Inverted A is indeed mentioned by some as extremely rare but I do not THINK it is as rare as that. I have seen some attributions as R3 or even R2

In my opinion the letter looks exactly like the one in this piece where it is not mentioned to be (but I think is ) an inverted A

https://coins.ha.com/itm/great-brit...tion-120115#

and also this piece (different date different V but same inverted A type.
https://www.thecoinking.co.uk/products/23t259

So I think you are right but I do not believe that it is R5 as I personally have one example of this exact one although the silver is mined from the westcountry so it also has roses as well. I think for it to be R5 it needs to have a couple of the other variety bits too but don't hold me to that one.
Regardless its a nice find and probably a true expert will have more to say than this and may even correct me on the rarity