These are replacement cross heads for  Minitrix A3 (4-6-2), A4 (4-6-2), 7P (4-6-2 Britannia), 9F (2-10-0), US K4 (4-6-2) and US Decopod (2-10-0) steam locomotives.

These are a direct replacement for the original.

They come in a packs of 2 and 4 is are available here and here.

You can find posts about replacing the cross head here and here.

The kits are printed in the Shapeways FUD material which will require cleaning upon delivery, please see my FAQs for more information.