Partially Brass Plated 1997 Hong Kong Steel 50 Cent?

Hello all.

I have a 1997 Hong Kong 50 cent coin that I believe is missing most of it's plating. I would not have noticed it if I wasn't stacking it with the others.

I have shown pictures of the error coin alongside a regular variety. The error coin is mostly a dull silver color, with a hint of golden coloring along one edge.

I am most interested in if this is an as-minted error or post-mint damage. Krause lists it as brass plated steel.

If a true minting error, I am wondering how it happened. I do not understand how these coins are plated. Usually I think of 'plated' as electroplating where each coin would have to be in contact with an electrode, but I have heard talk of 'washes'.

Thanks,
-wheatiefan