Back in 2013, Ron Nelson provided me with a few details about the unreleased Designer Master 2400.

"One of my last projects at the German bought Fidelity was designing an ARM2 processor into the Designer housing, which became the Designer 2400. I did the hardware and software test program and built a prototype and sent it to Germany. I know nothing beyond that. The Designer 2300 with a 3001 (?) processor is after my time".