Not sure if this is the right place to post this but I'm curious as to when the AC spark plugs that didn't have the four green rings around them were made? I know some had them and some didn't. I've seen one ebay post showing a set of no green ring AC spark plugs from the 50s. I have these plugs that came out of my '49 Plymouth and I'm trying to find out when they were made. Here is a picture. Thank you! Any info is appreciated.