On your last image, the flats are the area from the left side of the image back to the end of the tubeset. There will be touchmarks there. It is the area where the flat portion of the tubeset interfaces with the receiver/water table. Can you say the the Nitro stamp is in script? The "Crown" over "G" touchmark means that the tube experienced proof with a solid projectile.

Kind Regards,

Raimey
rse