In a violent life and death struggle where the Dallas Police were fighting for their lives, of course Oswald shirt would be tugged and the arm that was holding the revolver would be seeing a lot of action.
That's a good point, I don't recall seeing any photos that day of any Policemen wearing a short sleeve shirt. And here I only see a bit of what could be the brown shirt?
JohnM
Here's the SWP with officer Bentley as he and the funny-looking guy dance cheek-to-cheek* in the bright sunshine (which in turn reveals the brown shirt, lightened somewhat in the sun)
* Maybe that's what the funny-looking guy meant when he kept exclaiming 'I'm not resisting!'