I am not familiar with exactly how this forearm locks into place. I am believing it has some retaining cam or pin as shown in the third pic. I would take a sharpie and outline the part that inserts into the rectangle shown. tighten the handguard in place. disassemble it. look for areas, high spots where the material is not allowing for a full lock up. (Those being the high areas that caused a shiny spot on the insert.) I am believing this is somewhat like a shotgun forearm. I have seen some that have too much material and due to changes in temp and humidity the lock up cannot fully lock/cam into place.
I hope that helps. I do believe a careful inspection of the contact surfaces will lead to the solution.

 

The only way it locks into place is a barrel band screed in front which does little for the back end

 

Is that a spring clip riveted at the back end of the handguard that is supposed to snap over and around the barrel near the chamber? It also appears the the forearm has moved forward of the receiver. I have seen such clips as I am describing used successfully on certain models of Mausers. (the Iranian Mauser Carbine made by CZ comes to mind) They were snapped on at the rear, and held in place by the barrel band. The square hole in the handguard might need to mate with a male counterpart on the barrel to keep it from moving forward under recoil. That, working in conjunction with a proper fitting band and spring clip sounds like sound engineering to me. Perhaps you can find a reprint manual of the military version of these guns. The version most made (300,000) went to Russia in 7.64X54R. Shore is a sweetheart lookin' gun though. I hope you can get it right. Perhaps Browning USA or Winchester might help. AC

 

According to the owners manual for my reproduction Win 1895 carbine, there are supposed to be two clips somehow associated with the upper fore end hand guard, that hold it in place. It also says not to remove the forearm unless truely needed as it will scratch barrel (so I assume the clips are somehow attached to the upper fore arm hand guard itself, and secure it to the barrel), there is no schematic, and I've never taken the fore arm off, but they are supposed to be there. If they are missing, that could be the problem.