I uploaded a video presentation of what I'm talking about. This presentation was merely meant to be artistic and suggestive of what you need to do to tell there are, or have been Two Paul McCartneys. One active primarily in the early 60's, the other in the late 60's and beyond.
It's all about the length of arm. One has a longer one than the other. You can tell this by when they are both playing acoustic guitar.
Did you spot it? I believe the guitar is a Martin Standard D-28, though each one has a different design. The pickguards differ, as does the cutaway design from the neck, but the size of the bottom portion of the body is the same. I mean, you don't even need that reassurance, because looking at the overlay, you can see that Paul McCartney standing up, looks like he could be playing the guitar of the one sitting down. Which tells you another thing. Whether standing up or sitting down, your arm is only going to reach so far. Well, if your Paul McCartney TWO, because you can just reach the soundhole of the Martin. Whereas Paul McCartney ONE, if he extended his fingers, could probably place his most of his palm over the soundhole. And possibly touch the neck as well. You should also notice that Paul McCartney ONE's fingers on the fretboard look much longer than TWO's. That's because they are. That's probably why TWO does not play the same chords on Yesterday as ONE did back in the day.
What this tells you is this. People's bones don't usually shrink like that. Especially if you're at the age of 24, and then the next year your arms and fingers shrink just in time for your 25th birthday. That's pretty weird, and I imagine, could be kind of painful to have your bones retract so quickly.
Wait you say! In 1973 he has no problems reaching the soundhole. Well, that's because he's cheating int'he? Look where the guitar is. It's resting on his left leg. It changes the angle at which the guitar is played, allowing his arm to adjust position so he can reach that soundhole. As you can see, it looks like ONE is playing that guitar , but to play at that angle is the only way TWO has the reach that ONE naturally has.
You should be thanking me really. Or at least saying, you know what Dr.Tomoculus, you're right Sir.