I have some Hiwin HGH15 rails - if I recall correctly I have four 760mm long rails and two longer rails (1100-1200mm I think, will have to check), plus at least 10 bearings to go with them. All new, genuine Hiwin bought a few years ago before they started turning up everywhere.

Is that one THK rail you have available to swap, or two? If it's two then I may be interested for my X3 mill conversion.