In sewing patterns for childrens clothes from the 1940s through today, not much has changed in size standards, unlike ladies clothes patterns which have changed sizing a number of times thru the decades.
In Simplicity patterns from the 1940s, a Size 1 is Chest 20 / Waist 19.5. This is close to today's standard for 12 Months. Likewise a Size 2 is Chest 21 / Waist 20, comparable to today's Size 2 or 24 Months.
Layette patterns for new babies from the 1940s, typically consisted of a christening gown, bonnet, bib and jacket, were sold as "One Size." So there usually wasn't a differentiation for Preemie, Newborn, 3 Months and 6 Months like there is today. That tells me, by 1940s standards, a Size 0 would have been the only option for any baby under 12 months. And I guess that since a baby isn't actually walking in the bootie at that age, one size was close enough.
Attached file shows size information from a 1940s Simplicity pattern for Children's sizes 1 to 6.