The short answer is that it's not at the park and nobody who was there Sunday knew what happened to it. 

I'll be poking around to try to find it, but in the meantime, I did learn a few things:

According to a Travel Town brochure, a master plan for the museum was adopted in 1987 specifying that objects that were unrelated to railroads and transportation in Los Angeles be deaccessioned.

For example, the Korean War-era planes were transferred. An F7U-3 Cutlass went to the Navy museum in Pensacola, Fla., and a P2V-3W Neptune and F9F-2 Panther went to a museum near Fresno.

All the firefighting vehicles were transferred to the firefighting museum in Hollywood.

Obviously "The Last Argument of Kings" didn't fit with the museum's master plan. With luck, I'll be able to find out what became of it.

Two unidentified sailors en route to China pose with "The Last Argument of Kings" cannon, Aug. 23, 1947.

Here's the plaque, which was removed by the time the cannon was installed at Travel Town.