I'm not criticizing your friend, but I woun't use the source book that you have mentioned as a primary basis for identification. Like other materials on the topic of the Russian Imperial Army published in Russia right after the fall of soviet regime, it contains very fragmentary information, with a number of mistakes, with very poor quality of illustrations. I have this book, but can't find it right now, as I didn't used if for years - it was almost useless to me. For example, if the book covers the period of 1917-1920, why there are many illustrations of pre-1914 uniform pieces? Again, I would like to put your attention to the fact that shoulder-board is a war-time piece, while last fortress units were disbanded during the army reformation of 1910-1911; soldiers and officers of these units were used in formation of new infantry and rifles regiments.