There is a slight difference in the size of hands and arms from the early figures in the 70's.
The opening on the early figures hands was a little bit wider if you try and put a modern polearm into it they just simply drop through.
I used to think that this was caused by play and often put hands into hotwater to soften the plastic hand and close it to take the newer weapons.
Also if you put older one piece hands onto a torso with the new style inner frame they stand away from the body more and don't fit as good at the shoulders.