I"ve read that when Bill Ruger worked in Washington, D.C. -- employed as what I don't know -- he spent his lunch hour in the U.S. Patent Office, looking through gun patents.
From these he took the Iver Johnson, "Hammer the Hammer" safety for his later double-action revolvers, and the Nambu design.
But really, "stole?" A good idea is a good idea.
I'm sure the ghost of the man who invented the hammer -- the world's most common, oldest and unchanged building tool -- still grumbles that his design was stolen. The hammer, with its claw on the back to extract nails, dates at least to Roman times and probably much earlier.
When the Ruger .22 auto was announced in 1949, for a few years afterward people mistakenly referrred to it as the Luger. I have a complete set of American Rifleman magazines dating from 1929 to last month's issue, and it's amazing how, in 1949 and the early 50s, readers and even writers mistakenly referred to it as the "new Luger" or "inspired by the Luger."
But you're right. The Nambu was very likely the inspiration. The later Nambu was a good design. Had John Browning been aware of the Nambu design that fully enclosed the bolt, he might have tinkered with it.
I still think it's a good design. I'd like to see the Ruger Mark III in .25 ACP and .32 ACP as a small game gun. It's amply strong to accommodate these cartridges.
Heck, in .380 ACP it would be a good small game gun and a passable home defense gun. Kinda bulky for carrying concealed except in a shoulder holster.
Very interesting black powder pistol. Obviously patterned after the Ruger, ne' the Nambu.
How's it shoot? What caliber? What kinda pellet and saboted Super Flooper Jacketed Turbo ZX Mega X-Treme bullet ya use in it?