Zombie Mode Pieces System

Hello everyone,
so as i figured it out when i reach the 200 pieces i can get the option that i can exchange with 100 and i'll get the weapon permanent .. i did that like 4 times with the Arm. Warmonger Fist as u see in the pic and i got it 4 times permanent .. but when i reached 200 pieces of the AWM it didn't show me the option to exchange it permanently .. so what's the problem here ? is the Arm is the only weapon i can get it perm or its just the AWM that i can't ??
Pics links :
https://imge.to/i/NOa0q
https://imge.to/i/NOYoO
https://imge.to/i/NO9H0