What determines which attachments/mods are available at workbenches?
Edit: I mean the workbenches you can find near buy stations inside a running game.
I have a mission like "kill 10 enemies with iron sights, thermal and hybrid scopes in a single game." The issue with that is that all the scopes/sights available on snipers for me aren't scopes besides one. This means I can't do it solo unless I can somehow acquire an mcpr with these scopes inside the game.
I asked another guy and he seems to have had access to the raptor scope, I do have this unlocked but it doesn't appear at the workbenches. I have access to no scopes with a greater magnification than 4x.
Recently, I unlocked the agent grip and I can now see that in my workbench but it seems to have replaced either the hex grip, merc grip or pineapple grip. Regardless, before unlocking this I had access to some verticle grip and now I don't.
Any idea what determines what's available? My best guess is whatever you unlock last will replace previous things even if you unlock something relatively useless.