I'd simply check a few easy to measure dimensions and calculate each. Do several, as one need is to see if the scale is reasonably consistent!
I'd start with wheelbase and diameter of wheels...I think the 19" wheels would be about 20" due to the flange.
I can't answer your question regarding Scale, but these metal Hubley Model A Fords are nice when painted and built up. This impressive Diorama of a Ford Model A Assembly Line is on display in the recreated Model A Dealership built in the grounds of Gilmores, at Hickory Corner, Michigan. I believe that these Model A Fords are Hubley, but I can't confirm that. Take a look at this neat display. I think that they are 1/24th Scale, but am not sure?
They are bigger than the 1/24th scale models I have so I would agree with the 1/20th scale. Funny it doesn't say anywhere on the box or in the instructions what scale they are.
Seeing these models were made many years ago, they were pretty accurate for the time, and even today, rate as quite good, even though the art of model making is so more advanced. Hubley offered the Model A Ford in a variety of body styles. As has been mentioned above, there is no indication of the Scale on the boxes, but in light of the above comments and above picture, it is fairly safe to say that the Scale is 1/20th.
Here is how I did my Hubley Sedan. I find it odd how the tops of the front side windows tip down in the front. Would that be to avoid copyright infringement?