Pietta offers those as standard on the 1858 they offer with the color case hardened frame. I have a pair of them and those grips have a much better feel than the standard 1858 grips.
Pietta shows them on their web page on the stainless target also. Like all of the grips for these guns a set fit to one gun won't likely be a perfect fit on another.
True, and a shame really! I much prefer the looks of the regular grips on the case colored frame, and the checkered grips would be much more advantageous on the target model. But I'll live with it.