Picked this up for a buck, not worth a lot more than that in scrap. It weighs 8 grams. I don't know what the "66" is for though, if it were a Taxco piece, wouldn't it be at least TC-66? The details of it don't show all that well in these pics.
That was my first thought when I bought it, but it is marked .925 as well. The marks are very hard to see with the naked eye and I missed the .925 at first, but I knew it was silver. Assuming it is oriented the same way as the .925 mark, it is 66 and not 99.