how to mimic PNG8: operator?

[jmaeding]

I want to quantize an image to 256 colors to get an 8 bit palleted image.
I know prefixing the output filename with PNG8: will do it, but what is that longhand?
I tried this with no luck, gives a 24 bit fill color image:
orig.png -dither FloydSteinberg -colors 256 -type Palette new8bitindexed.png

That quantizes the colors perfectly, I compared to corel PP, but does not index them.
I tried to read on the -map option, but its not clear.
any help appreciated.

[glennrp]

Please try ImageMagick version 6.6.1 or later. It works for me.