Hi Starbie. The best explanation that I can get on that is the white line you see in Paulus Somni is that it is a ray from the impact of the crater Proclus. (Page 93 of THE MODERN MOON by Charles Wood).
I haven't looked at it myself lately, but will try see it tonight if conditions and Moon phase permit. Proclus has to be sketched at Lunar sunrise and Lunar sunset anyway for the Lunar II program.
I hope this helps. I got both the Rukl atlas and the Charles Wood book at the same time, but hardly use the Modern Moon book anymore. It now keeps an honored place in my library. Its handy for questions like this though.