Seems like the guys at Forgotten Weapons have been making some of their research online and found this forum
Only a couple of mistakes that I noted after watching it: 1891 handguards were indeed changed for those shown that have a step that fits against the rear sight base, but those rifles of the Peruvian contract originally had the long '91 handguard, not the shorter ones described that were used in the earlier rifles.
Also receivers were indeed re-finished when the '91s were rebuilt, but they weren't originally "in the white". When upgraded for Spitzer ammo, but ALL of those that went through this process got a new Sig barrel, and not "most of them" as is stated.
Finally, rifles weren't stored in State depots without their bolts to avoid theft; that was only done when they were taken out by troops in war games and rifles weren't properly stored inside a military installation, but inside a tent or so under soldiers' guard; anyway most mismatching probably happened when they were taken apart for cleaning.
Still great video; hope they'll eventually make the same for the Vz-32 and the FN-1935 too!