The weight is way off even with circulation. The FM is the super rare variety. The FF is the regular which this is. Silver melt is $14. If genuine, this coin in VG, NGC says $75. (Their prices run high, but I would be at F/VF on this anyway.) In VF they list $150. Now the weight is supposed to be 27.0674g. And while there is a little fluctuation there, 3 grams seems like a lot for a heavily counterfeited coin. Wait for expert opinions. I have to defer to the consensus. Weight is often a good indicator and 11% missing seems excessive.
Looks like shipwreck effect, which would account for the porosity and loss of weight. If you can find a way to check the silver content to see if it is 90% silver then you possibly have salvage from the El Cazador. Most of the recovered 8 reales from that shipwreck were dated 1783.
