It wasn't caused during minting. Unless you speak to the person who made the marks, you will probably never know for certain what caused it. I cannot see such marks adding historical value to the coin so in effect, it is a damaged coin worth no more than face value being 20c.
i wonder that people are doing on these coins, testing out on cheap metal? so that they dont ruin actual sheet metal?
Nah. Just overly active people. Once had a young guy in the workshop at work who just couldn't keep still. If there was a drill handy while sitting at the work bench he'd drill holes in the bench just to be doing something. I can just imagine him using a spring punch on a coin because there was a punch and coin handy. He ended up joining the Navy which was the best thing for his excess energy.