"The disk is cast or, more likely, cut from a sheet of bronze which appears to be of the same alloy as that from which the handle was made.

Though he's not had the benefit of a spectrographic analysis to prove it, I suppose that Benediti is assuming that both the mirror and the handle are from the same bronze alloy* --presumably from the color (or colour) of both the bare bronze itself (wherever/if it may be seen on both elements) and that of the patina.

Mixing up batches of bronze was probably far from an exact science, so there would inevitably be variations in the alloy content from batch to batch; and we have seen in earlier descriptions that certain alloys are more prone to "excessive" oxidation, which manifests itself as occasional spots of thicker encrustations (rather than as merely a smooth patina).

The assumption that both parts of the thing were made of bronze of identical alloy contents is, of course, a quite reasonable one --it is highly unlikely that the smith would mix up two separate batches of copper and tin to fabricate both elements of a single (compound) object.

It is not clear to me why he believes that the mirror was "cut from a sheet" (not a "leaf") rather than merely cast in a round mold (which could then be further smoothed through abrasion. Perhaps there are irregularities in the circle --trimming marks-- which suggest that this was the way the thing was formed. In any event, since he doesn't give his reasons for this assumption, the hapless translator is not obligated to speculate on what those reasons might have been.

*Tony is quite right that all bronze is an alloy; however, I believe that the use of "bronze alloy" here is not redundant, since it refers to the *specific* "alloy content" i.e., the relative amounts of copper, tin, etc. which made up the specific bronze alloy in question. (We can't use "allodial" because than word means somthing else.)

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Note added at 19 hrs (2017-03-28 13:24:49 GMT)
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A bit of a change for clarification purposes seems necessary:

"The disk is cast or, more likely, [it] *was* cut from a sheet of bronze which appears to be of the same alloy as that from which the handle was made."