From here: Masters Guide for Pokemon WOTC Corrected Errors & Test cards
“Matchprint” cards
These cards were used for color proofing before their set officially went into print. Where FPO cards are all about size and placement of the artwork, Matchprints are about the color. Two non-holo Expedition partial sets have been circulated, sourced directly from a print company contractor of WOTC. Of these two sets. Barring another Matchprint card discovery, these remain considerably tough to find.
Front and back:
They use non-cardstock material, say “matchprint” on the back and are square cut. As mentioned in the thread I linked, there is basically only two public copies of most of the non-holos