Welcome to TDH Zeever Fett!
There's been a LOT of discussion about the Mystery helmet as well as several recastings which tend to dilute the discussion. Once recastings appeared, the shape or quality of the helmet sometimes changed. So although a helmet is called a "Mystery" helmet, it may in fact be a 4rd or 5th generation helmet. I have one of the originals from SgtFang, which I love. The Marrow_Sun (version 1) Mystery helmet is also very nice. Here's a search function link that will answer most if not all of your questions about the Mystery: