You will need to use the spacer as this is not our A5 system.
Yes, in this scenario you will need to use the spacer. If you want to get the benefit of UBR Gen2 longer receiver extension and negating the need for spacer use in the process, you’ll need to get the Vltor A5 buffer and spring, which bridges the reliability of a rifle length gas system and compactness of that of a carbine setup.