40 years ago I wrote a paper on nuclear weapons. My memory isn't perfect and the sources are 40-50 years out of date. But here goes.

The US built a massive manufacturing pipeline to make nuclear weapons. Little boy was a once off. Fat man was basically a functional prototype. And the manufacturing pipeline was running and kept running after the war. How much weapons grade plutonium they had left over after they bombed Nagasaki is kind of a quibble.