"I am wondering if some of the older common strikes could be turned into errors too?"...

There's no mystery involved in punching out the center of ANY silver strike (old or new) and re-inserting it at a different "off-center" rotation alignment. I don't buy the "error" bit.

Anybody who is considering turning in extra strikes can easily "realign" the center before redeeming them so that others can have a thrill of finding such "errors".

Just my 2 strikes worth...