The folded buttstock is described as:

Folded for mobility, but gives less stability while aiming.

This can be found in south western Montoyuc in the corner of the province. The accessory case is inside a railway tunnel in the mountains. There’s no resistance here.

I definitely recommend taking a helicopter to get here as it’s really remote. You can land on the flat rail track line and just walk in and take the accessory.

There are a lot of folded buttstocks in the game for ARs and for SMGs, and I’m afraid I don’t know which ‘AR and SMG’ this one is compatible with, but if I had to guess, it’s probably for a shared weapon platform such as the AK-12 or AK-47 and the 9x19VSN perhaps.

I don’t feel disadvantaged by low mobility weapons when I play, so I much prefer stability while aiming. So I’d rather stick with the extended buttstock you get with the gun anyway. But pick this up to give it a go, to find out which AR and SMG it affects or because you’re going for 100% in the game.