The tires on each line were the same size, were formed in basically the same molds, but were numbered differently to reflect, first, the increase in recommended rim widths, then to reflect over all carcass width rather than shoulder to shoulder width across the tread. If you will notice, some real old tires will have both sizes on them. It should also be noted that the tire industry adopted a more aggressive tread pattern about the time the first size change went into effect. Also you must remember that rear tractor tires evolved from existing sizes used on aircraft. So there may have been a slight increase in physical size when molds used exclusively for tractor use came about.
Most restorers are aware of the second change, but few are aware of the first change.
You squeeze those beads together by eight inches, the tread shoulders will probably come in that inch and a half. I hope I have shed some light on rear tractor tire sizes, and their evolution over 65 years or so, and if you thought some tractors at shows looked odd because the rear tires seemed small, you were probably right. The restorers used the best information they had, and should not be faulted for this common mistake. Although I am no expert, I am someone very familiar with the rear tire industry, and have come to these conclusions from studying tire sizes on old tires for a number of years.