Here's a little more information about PTO standardization -
Way back in 1926 the Farm Equipment Institute (FEI), now called the Association of Equipment Manufacturers (AEM), realized the need to standardize all the various PTO speeds, sizes, and locations that existed. All the various tractor manufacturers had their own favorite designs that all differed and it caused real problems for the independent equipment manufacturers. So they developed what they believed a standard PTO arrangement should be and proposed it to the ASAE (American Society of Agricultural Engineers), which adopted the original standard in 1927.
The standard was titled ASAE S203. It was tweaked somewhat in July 1928, March 1931, and August 1941 to reduce some of the variations in the original standard. That final version resulted in a 540-rpm, 1.38-inch diameter shaft with six splines, located on the tractor’s centerline, and 14 inches ahead of the drawbar pin hole.
The original 1927 Standard called for a speed of 536 +/- 10 RPM. It’s unclear why this exact speed was chosen, but it appeared to be the best compromise between all the various speeds available in those days. For example, a study in 1929 showed pto speeds ranging from 515 to 745 rpm across a range of 35 different tractors.
That standard was in continuous publication by ASAE (now known as ASABE – American Society of Agricultural and Biological Engineers) until early 2011, when it finally discontinued as ASABE decided to adapt a number of ISO (international) standards pertaining to front and rear mounted PTOs.
Apparently Deere didn’t get on the standardization bandwagon until a little later in the game. Field Service Bulletin FSB 148-S, dated June 1944, described a number of “kits” that were designed to retrofit a number of existing (old) tractors to cause them to essentially conform to the requirements of the Standard. The FSB noted that the 1-3/4” diameter shaft of the model “G” was recognized by Standard S203 as an acceptable alternate size.
I don’t know the evolution of all the Deere tractors, but I do know that about that time, the model “H” tractor PTO (“Power Shaft”) was changed from a 1-1/8” diameter shaft to the common 1-3/8” diameter shaft, and there was a change to the drawbar at the same time in order to conform to the ASAE Standard 203.