Soo, all in all it seems around 60 different Klingon forehead pieces were created and re-used between TNG season 1 and ENT season 4. In the remaining posts I will introduce you to *some* of them to see, which Klingons from different series and episodes *might* be related.

This might be terribly boring so please be warned.
I assure you, though, nobody has ever done this.
I did this research for an article that might be released some day. I have archived and classified every single Klingon, even unnamed background characters.