Hey comrades, this will help explain the little caveat that is the Underfolder stock adapter for the MG100 series of the Bloc System. The kits and adapter themselves do not include the real steel underfolder assembly, because they're hard to come by, and would be difficult logistically for me to keep in stock. That said, for those that choose to go on a hunt will be treated to a real cool looking and functional stock! I have a version on my personal rig that allows me to fold it over a 10ci HPA bottle and the 20rd DAM mags, and has perfect mas clearance!
Anywho, AK parts are hard when it comes to nailing down real steel origin countries, model/variant numbers and the exact differences between them all. I designed the AKMS/AKMSU underfolder assembly around what I believe was a Polish surplus underfolder assembly. However, I have since found Polish stocks that are like the Hungarian or Yugo variety, which won't work. So instead of saying "Polish Underfolders work" or Hungarian/Russian/etc... I'll just post pics of the type of assembly that you'll need to use an underfolder, as well as at least one link I can see that has them in stock.
So, above is the assembly I used. It only has one "key" and the rest is a smooth shaft. This one works well because it allows for more meat on the stock adapter. There is a second style in the real steel market that I've seen, it has TWO "key" sides, and it easily identified by the addition of a single part, as shown below:
If you are sourcing an underfolder assembly, DO NOT purchase a stock with the above components. It might not work (I actually haven't tested this, maybe it could be assembled without that extra "key", but I don't want people to risk it). Now, another thing to keep in mind is these stocks are commonly sold incomplete as surplus stores don't always have access to complete packages. Here is a picture of every necessary component for your underfolder stock assembly:
If it comes with extra pins, that's ok. You only NEED one, but a second would secure the fastening nut.
The other thing to look for is a flat arm assembly if you'd like the underfolder to fit over HPA bottles installed directly on the MG100's ASA. Round arms like the above pics will not fit over an HPA bottle without being milled out a little.
Here's a link to a known in-stock (as of Oct 2019) supplier of the correct, complete underfolder assemblies:
You may be able to piece together a cheaper one through GunBroker, but it's not reliable.
Lastly, if you have any questions contact me via firstname.lastname@example.org.
Enjoy your build!