The folded buttstock is described in the game as:

Folded for mobility, but gives less stability while aiming.

After being bugged and totally unavailable for a long time in the game, this has finally been patched – as of (I think) Title Update 5 – so you can now, finally find this accessory in eastern Pucara province at a small cartel outpost near Pilca Village. The accessory case is sitting out in the open at the western edge of the area.

There are only a few enemies here, including a Lieutenant, a Captain and a couple of heavies with a handful of sicarios.

I approached this at dawn from Pilca Village to the east via the road. I took out the checkpoint guard first, then the two heavies, Captain and Lieutenant with a sync shot, leaving just the sicarios which were easy.

The Stoner LMG A1 comes with the extended buttstock as standard, so collecting this will grant you the folded version, which reduces accuracy and improves handling.

Whether you want to use this or not totally depends on how you play with LMGs or the Stoner in particular. But if you watch by Best LMG class video you can see how I set up the Stoner to be used all day long and depending on the range you like to engage at may depend whether you find handling or accuracy more of a benefit.

If you want to make use of this at shorter range, then you’ll probably find the handling increase from this folded buttstock worthwhile.