Looking closer at the photo provided by Jerry, I see that the surfaces of the
moveable bayonet lug that would contact the underside of the barrel form two parallel lines (see arrows in the photo below. This is due to that smaller radius curved surface of the lug. These parallel line surfaces appear to be somewhat rough to me, although the photo is not really clear enough to show this very well.
It appears to me that the contact between the
moveable bayonet lug and the underside of the barrel could create some rough spots on the underside of the barrel (which would be in the form of two parallel lines). When Day needed to apply the tape to the underside of the barrel, all he needed to do was slide the bayonet lug away from the print (towards the muzzle end of the barrel. The rough spots (in the form of two parallel lines) could retain some of the fingerprint powder. And those rough spots would show up on the lift as two parallel lines.
Thank you Jerry, that was some very good detective work and research.