Fella's;
In all actuality, the parent cartridge is the 8X57mm Mauser. That's where the .473" head diameter, extractor groove dimensions, and base diameter measurements originally came from. The .30-06, .30-03, and therefore the .270, share those critical numbers that Peter Paul Mauser first laid down.
900F
And the parent case of the 8 mm Mauser (7.92x57) was the M/88.
The M/88 was also the case that was the model for the 30-03. It could not have been the 8mm Mauser as it was released in 1905. It was the 8mm using a 150gr spitzer bullet that made the 30-03 with its 220gr round nose bullet obsolete and was the impetus for the .30-06.
The real disagreement is not about the parentage of the .270 Win, it is about whether the .30-03 is a seperate cartridge or just the first stage of development of the .30-06. The thing is, almost no one has heard of the 30-03 or even cares that it once existed. IT was around for 3 years. Only two rifles were chambered for it. The 1895 Winchester didn't sell well and the almost all of the '03 Springfields were rechambered to 30-06.
So most writers, editors, and many historians rather than recognizing the 30-03 as a distinct cartridge, consider the 30-03 to be a preliminary form of the 30-06 with the 30-06 being the final design and thus the parent cartridge of all that came after. That would be fine if the 30-03 had been strictly a military round that became the 30-06 in final form. But it was not. It was released commercially with a commercial rifle chambered for it for the civilian market. That makes the 30-06 a separate cartridge from the 30-06, albeit a short lived one.
To make matters worse, the is/was a .270-06 wildcat. It
is a 30-06 necked down to .270, but other than the difference in case length and neck length, there is no difference in the case, but the '03 and 270 chamber is cut with a longer (2.880) throat than the '06 chamber (2.7442) and a 270-06 in a 270 Win chamber will lose accuracy and suffer greater throat erosion. Same as a 30-06 in a 30-03 chamber in a 1895 Winchester.
It's real simple: the 30-03, .270 Winchester, and .280 Remington cases are 2.54" long. The .30-06, .35 Whelen, 338-06 A-Square and .25-06 cases are 2.494" long. We know for certain that the 30-03 was shortened to make the 30-06, We know for certain that the .270 Win case is 2.54 inches. If they started with the 30-06 to make the 270, the first thing they did was turn the 30-06 case into a 30-03 case, then neck it down to .270. The moment the 30-06 case is turned into a 30-03 case, the 30-03 becomes the parent case of the 270.
But the M/88 is the granddaddy.