There are plenty of information backed by documents about this and other variations in two books by researcher from Ukraine Alexander Yuschenko. These two books are must have for any serious MN collector (I don't consider myself MN collector, but still find these books very useful).
1. "M91/30 Rifles and M38/M44 Carbines in 1941-1945"
Review on Forgotten Weapon. This one is almost impossible to find these days.
2. "Model of 1891/30 Rifle and its Variations. History of Development, Production, Modifications and Use". 700 pages.
Link to the publisher web page. This one is unfortunately in Russian, however all illustration captions are in English. There might be translation of this book in the future.