So… I got hold of these almost identical MPs. One set with BOXY molded on the body - M5-201 BOXY and the pastel hued COLOR M5-201CR that appears to have the molding blanked off. In the second pic you can just about make out the oval seam line on the COLOR version.
Both were released at the same time and feature in the 1988 Mitsubishi catalog, of which I only have snippets of. The BOXY double knock M5-330 is also on the page… but no non-BOXY version was made.