They were used on Russian Side and underfolders. This is from Tantal's Projects page 9: "This plate is used on the AKMS as well as all AK-74 folding stock variants (except AKSU) and is actually spot welded to the receiver. It helps to make the receiver more rigid and keeps it from beign damaged if the rifle is dropped on the grip when the rifle stock is folded." http://tantal.kalashnikov.guns.ru/pro9.html
yes it is a grip plate i myself purchased one when ordering my last kit from todd at orf its quite sturdy i have yet to put said kit together as xmas has cramped my project money will advise on its use when i get it done
This plate was designed to be used on the receivers that are 1mm. Of course being a military firearm these were subject to massive abuse by untrained 'soldiers'.
I am sure that your gun, my gun or anyone's gun on this board will never suffer being dropped on repeated occasions and would need this, especially on the 1.2 and 1.6mm receivers. Although, I would put one on my gun if I get a 1mm receiver that I was building as a folder, just because I want the 'authentic' look. But it's sole function would be cosmetics.