If I recall, the LOP (length of pull) on the fixed stock is 13 1/2". LOP on the underfolder is more like 13 3/4" - 14". A cheek riser - cheap and readily available in 3D printed form on Gpost, or a more expensive Alu. model from Samson Manufacturing - makes the underfolder a whole lot more comfortable to shoot, and I would consider it a necessary upgrade if buying an underfolderBuild quality wise the LMG has the best fit and finish, and are the most accurate.
For its original intended use, the Side folders with sleeved barrels are the best for its length of pull and balance, mimics its Soviet contemporary.
The Underfolders are about 1 inch too long with lack of cheek weld, but has a superior barrel that is threaded.
All the fixed stocked models have a bad stock comb height, which was put together as a commercial after thought, poorly executed and was never military adopted for a reason unlike the Side folder.
People who complain about the short LOP are shooting these like fudd static bench guns, which is not how these are intended to be used.
Other things to consider are your planned optics choice. As it stands, the underfolder is incompatible with the AK style side mount rails, which act as a saddle for the optic mounts. Only optic mount choice for the underfolder is the Delta Dynamics (Xfiretactical) red dot mount which is now sold out - has been for many months - with no plans for restock apparently. They do pop up occasionally on resale sites, but probably inflated prices, and still limits you to just a red dot without further modification (adding a longer pic rail to top would allow the use of more varied optic choices).
My preferred config was a fixed stock rifle model with OEM stock removed and a DD 1913 rear trunnion with an aftermarket folding stock on the back. Allows the benefits of the fixed stock model optic mounts (preferably MM side rail and full length MM rail - allows the biggest range of optic choice), with a stock that has the LOP and angle you are comfortable with.
As a side note, I believe all Type 81M models come with the heavy barrel found on the LMG model. That includes both the underfolder and the fixed stock models. All 81M barrels are threaded with M16x1 thread and come with a thread protector as standard.
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