Re: Picked up new Atocha coin - maybe a 4R?
I saw this one on eBay... the weight piqued my interest. I do concur with DiveWrecks - I see nothing to suggest that the coin and Fisher cert aren't legit.
However (assuming that weight is correct - weigh it on a dig. scale if you can), is this perhaps a 4 Reales piece that the Fisher people mistakenly called an 8R? I've seen a few Atocha pieces where it seems likely/certain that the listed denomination is wrong based on weight vs. remaining detail, and plenty more where they missed a visible assayer, or even digits of the date. They certainly didn't spend as much time staring at each individual piece as we collectors might.
For having so much remaining detail on the shield side (plus the cross side isn't completely gone either), and not appearing to have lost any particularly large chunks around the edges, 10.5 seems way, way low (from a max. weight of about 27g). Also, from your pretty good side-view pics, the edges don't appear to have been worn razor thin like on pieces that have lost a lot of mass to sea-wear. I've seen 8R pieces with more pronounced sea-wear, like heavily sea-washed "razor" Santa Margarita and Consolacion wreck pieces, or really eaten-at Maravillas pieces, weigh considerably more.
These cobs, of course, WERE stamped with the denomination to the right of the shield... It is blobbishly visible on this piece. You can see part of the circle above traces of vertical lines... but it isn't clear enough to see whether it was the "IIII" for "4", or "VIII" for "8".
The way to check is to measure it... I've attached a pic of an about average-sized 4 Reales (slightly oblong) compared to an avg. 8R. This 4R measures approx. 31mm x 28mm; the 8R is 35mm x 36mm. How do these sizes compare to yours?