X1 was a top sealer/dressing sold by Tandy a few years ago - it was a follow up of Darke's dressing.
It had the advantage of doing a very subtle "antiquing" effect on tooling.
GOOD NEWS: Realeather now sells it as One Step Leather Dressing! We do not have to be without it any more.
On this piece I first dyed the darker patches with water-based dye; then applied the One Step dressing on the whole piece:
You can see how it only enhances the burnishing that happened during tooling (look at the border stamping).
I recently experimented with a mixture (50/50) of X1 (that I still had left from before - I bought a whole box full when it was discontinued) and Eco-Flo Hi-Lite Stain. Before:
This video shows the effect it has and how easy it is to apply: