Just found a stash of old leads hidden in a drawer, among them there are a couple of late '80s or early '90's Sheaffer 0.9mm leads that have "F15" as the only identification. They come is blue plastic containers with a yellow cap. I would like to know if anyone here has any clue as to the grade they could be? They are pretty firm and write somewhere between a 2H or 4H grade (depending on which brand you compare them to). Thanks in advance!