Galvano refers to electroforming, which is basically a really thick electroplate.
A rubber mold is made of the helmet plate, then the mold is painted with conductive material.
The mold and a piece of copper (anode) are wired to accept direct current and suspended in a container of acid.
As the acid dissolves the copper, the electricity draws it to the mold and builds up copper deposits.
It may take several days to build up enough copper to form a helmet plate.
As noted above, the reverse of the helmet plates made this way often have a rough, nodule-studded appearance.
The front will sometimes have a pinkish tint as the copper begins to show through the silver or gold plating.
Depending on how well the electroforming is done, these "galvano" plates can be pretty convincing at times.
Any flaws in the mold will appear in the finished surface.