Nice pickup.
And just a pet peeve. I don't think it's useful to call this the first appearance of "SA Atom", because the term first SA- is usually used when a GA hero makes a come-back in the SA. for example, when Captain America comes back in Avengers 4.
For Atom - this is not the first in the SA, its the first ever. It just confuses things that there happened to be a superhero in the GA with the same name. But these should not be confused as they are two entirely and unrelated superheroes.
As such, Showcase 34 features the first ever appearance in any age of this superhero.