The pure breeds still ruled in the 40s, but there were traditional simple crosses. This was a transition period of sorts. The NHs you became interested in were admitted in 1935, and was a commercial breed, bred for very commercial reasons. Into the 50s broilers became crosses etc. Reds (not as we know them, or Standard bred) persisted, as did Leghorns. These two changed over time, and especially as feed efficiency became more important. They were intentionally bred to be smaller.
The transition was not immediate, and there was no clear defining moment.
It was a period of controversy. Contrary to popular opinion, the pure breed adherents were very concerned with production. It was that they were also concerned with breed identity. What was winning laying trials were much different than what was commonly recognized as good examples of the breed. They were very commercial birds, bred for very commercial reasons, and there was absolutely no loyalty to the breeds themselves. It was all about numbers, not all that different than today. We just became better at it over time.
Not unlike the Leghorns Walt remembers. Leghorns still rule, but they do not represent the breed well. Leghorns ruled the laying houses then and now, and for good reason. So we still use "pure breeds" commercially, even if they are crossed strains. Just as much vigor can be realized between strains, as breeds. It is that they are no longer respectable Leghorns. Leghorns, well bred, then and now, are beautiful and productive.
Standard bred Leghorn hen weights are 4.5lbs. The commercial Leghorns end up about 3.5lbs. Barely large enough to lay large eggs. Just 18oz larger than some bantam hens.