IMO the M1891s that were produced in France in their original configuration and the M1907 carbine are two of the rarest Mosins out there.
I have read, but please don't ask where, that a few U.S. Magazine Rifle calibre 7.62mm Model of 1916 rifles were fitted with Pedersen devices. These, if they ever were, would have to be very high on the list.
It was an article in an older Gun Digest, which also featured a 1917 Enfield modified to use a Pedersen Device.