Bill, I took some measurements of the nipple.
OAL: ~ .67" (thread length (journal?): .21", base and cone length (or perhaps, height": .46")
Thread diameter: .245" (however, one can tell the threads have been rounded a bit, so I expect they measured .250" when manufactured... I wonder if the tap they used to thread the breech for the nipple was a bit worn?)
Also, I am fairly sure threading is 28 tpi. My gauge seems to indicate so. I have a photo of it, if I can get it to upload to photobucket.
Interestingly, the vent hole in the bottom of the nipple is somewhat erratic. Not entirely round nor centered.