That fourth Marx was a pretty smart guy...

 Zeppo owned a company known as Marman Products Co., which machined parts during World War II. Marman Products created a heavy-duty band clamp, which saw use in various applications, including securing atomic bombs in B-29 bombers like the Enola Gay. Marman clamps are still utilized in many modern vehicles, as well as in space vehicles like the Cassini orbiter.

Zeppo also co-invented and patented a wristwatch designed to monitor a person's pulse and sound an alarm if the heartbeat becomes irregular. He also invented and patented a therapeutic pad for delivering moist heat.

 In 1948, Marman Products developed and marketed a short-lived 2-cylinder motorcycle brand. These endeavors showcase Zeppo Marx's inventive spirit and mechanical aptitude that extended far beyond the stage and screen.

PS - It was his wife who dumped him for Frank Sinatra...