Note: User Iwaniw, writing in August and September 2008 on the Forum Classical Numismatics Discussion Board, says (as amended): "Milne lists two different obverse types for year 8 tetradrachms:

b2= head right laureate, drapery in front
b3= head right laureate, aegis in front (rarest obverse type)

The Isis bust right is listed (in Milne) with obverse b2 with crescent in front (Milne-1049); Milne-1058 has aegis on the obverse and the wreath ties are the same as a Dattari specimen. I suspect your type has aegis in front, but I am not an expert in identifying aegis.

I do not recall seeing an Alexandrian plated tetradrachm, but I have two coins of year 8 which have the same obverse and reverse dies – one looks silver, the other one bronze, so I expect the alloy varied a lot.

I do not have time now to post my year 2,6 and 8 Isis bust coins for comparison of the headdress, but Milne lists all his specimens with disk and horns."

(From a subsequent post, again as amended:) "Your coin is Dattari-1398, Milne-1058 and Geissen-855. The Isis reverse looks a bit strange. The two large dashes which seem to be part of her headdress are unusual. The rest of her headdress looks normal and is described as disk and horns.

The obverse with aegis appears to be a series of tetradrachms in Hadrian's year 8. Geissen lists 9 reverse types which have an aegis on the obverse. Milne only lists 10 types. The total number of types with the obverse aegis is at least 14 (not a thorough check done by me).

Why the aegis only seems to appear on year 8 tetradracms, I do not know.

My die matched coins of year 8 are of Isis Pharia, which have a crescent on the obverse."

(Iwaniw amended his original post after further research, and I have incorporated his amendments.) I am pretty sure that this specimen has an aegis on the obverse. The appearance of being a fourrée is probably just the result of differential leaching and corrosion. The "large dashes" on the reverse look like the original cow horns of Hathor which became part of Isis' headdress, but it is odd that they should be combined with a fairly standard small horns-and-disk.