Identification Mark on ball Rule 6.5

When playing in open comps run by the SGU , on the 1st tee you are always asked to mark and identify your ball.

Yes, but they can't enforce it and there will always be at least one bloodyminded individual who knows that and insists that he won't do it because he doesn't have to.
 
Always mark my ball with 2 black dots and a capital H underneath along with a black line.

Played today and my opponents knew what ball I was playing and the ball ended up in the rough. My opponent found the ball with the distinctive marking and knew I was playing a Titleist. He could only see the 2 black dots, line and H

He called the ball and when I arrived I had unintentionally changed balls and it was a Bridgestone, same markings.

There may have been say a 1 in 10,000 chance of it not being my ball.

What is the ruling please?
If you still had the Titleist ball in your pocket as evidence that you had changed balls, what was the problem?
 
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