The folded buttstock is described as:
Folded for mobility, but gives less stability while aiming.
This can be found in eastern Caimanes province, to the north of a lake. The accessory case is inside a Network Relay Station side mission.
Approach this like any normal network relay side mission. I have a separate guide on how to ace these every time. When you’re done with the side mission and mopped up the enemies, the accessory case is on the top floor, accessible via the exterior stairs.
This accessory gives you the folded version of the buttstock for compatible shotguns, which is… the SPAS-12 only. I normally don’t feel disadvantaged by low mobility on weapons when I play, however shotguns are best suited to indoors CQC like the Network Relay Station you get this from, and at short range handling definitely matters.
So if I equip the SPAS-12, I’d use it for these kinds of environments and I would definitely want to equip this.