[whym]

Are these public domain illustrations in the United States because they are works of the federal government or does the artist retain the copyright?

[trutannus]

You could also just check if the copywrite has expired. They are from 1975.

[ncmncm]

So, no. But anything can be released to the public domain, or under a liberal license, anytime, and there is no central registry to consult whether this has happened.

And, very incidentally, "copywrite" is not a real word.

[trutannus]

Correct, I was offering this as an alternative if the copyright has expired. It's no different from a CC or other FOSS license really. Should be stated on, around or near the work.