A 4.7-inch test mount would not make the 130 mm gun interchangeable

Would using a 4.7-inch Mark IX mount as a test bed make a 130 mm gun compatible with that mounting?

No; the Mark IX could guide development, but the 130 mm weapon would need a substantially modified mounting with different loads and elevation.

The 4.7-inch Mark IX mounting could serve as a test bed and design starting point for the proposed 130 mm weapon. It was nevertheless built around a different gun, ammunition load, and operating envelope. A useful dual-purpose 130 mm mounting would need extensive changes and higher elevation than the original could provide. Shared development ancestry therefore would not create direct compatibility with ships or fittings designed for the Mark IX.