Actually, plywood was used in PT boat construction during WWll.

It was used in building construction til the raw materials necessary were depleted and chipboard was developed was a substitute---but was not widely used until after the 50's.

The builder of your house may well have used boards(technically planks are wider than the 4"-6" boards) since that was the method that was used for many years before WWll.

Machines called planers or surfacers have been used for decades to adjust the thickness of lumber. I have one in my shop that can take an 8" by 13" beam and make it 1/2" by 13" is enough passes were made. In thousands of an inch and up increments.

So, by figuring what thickness was necessary for the boards, it was relatively easy to have a saw mill or wood processor machine the correct thickness.

Today, there are several different thicknesses of plywood, OSB(Oriented Strand Board),or underlayment available at lumberyards/home improvement stores. 1/8". 3/16", 1/4", 3/8", 5/16", 1/2", 5/8", and 3/4" are all easily found. Thicker ply/OSB is available by special order.

Using ply/OSB is much less expensive, much less labor intensive, and the wood does not move like solid wood boards. So, those are actually much better.

A method for the production of plywood was patented in 1797, but plywood has been found in ancient Egypt (even 200 years ago patent examiners missed prior art). The now-standard 4x8 sheet was introduced in the 1920's, but I frequently see plywood used in organs I restore dating from the late 1800's.

There are lots of ways to build up floor height, you need to look and see how yours was done. 3/4" plywood is a good bet.

My 1940 vintage kitchen had linoleum that appeared to match the height of the dining room hardwood floor next to it. When we pulled up the linoleum to replace it we found pine tongue and groove flooring that was the same width and thickness as the hardwood.

Bruce