This is a rather mysterious - or at least poorly documented - Mosin Nagant carbine variation. Made from an assortment of rifles dated from 1896 through 1920, these carbines were designed to fit Gulkevich folding bayonets. They have a barrel just slightly longer than a 1907 carbine, but were fitted with 1907 Carbine rear sights. They were also fitted with metal sheaths alongside the rear sights to retain the tip of the folding bayonet, as well as new sling swivels on the left side of the stock and barrel band.
Basically no documentation has been published explaining the origin and purpose of these carbines, but hopefully some will eventually be found. The best hypothesis I am aware of at this time is that the carbines were made by and for the St Petersburg Cavalry School.
Thanks to the anonymous collector who shared this one with us!