Perhaps it is a mower designed for a few early Kubotas whose pto turned backwards compared to today's machines. The blades may be different. The cutting edge and up twist to lift grass may be opposite to what you need when connected to your pto.
Study the pto rotation of both machines and also how the blades turn. The cutting edge of the blade needs to approach the front end (closest to the tractor) of the discharge shoot. Other wise the cut grass is thrashing around under the deck. When you back up with a mower designed to be pulled forward, the first part of the blade to touch uncut grass is far from the discharge which compromises its performance.
These are my quick thoughts early in the morning. Hope they make sense.
Dave M7040