There were a bunch of different carriers, depends on when your gun was made, I've seen brass, steel, aluminum and case hardened spools.

If your gun has a steel carrier (spool) it is most likely from a WW2 era manufactured gun, they started using steel in a few of them because there was a shortage of brass due to all the ammo they were making for the war effort.

If it's a newer gun then it's probably aluminum, they started with those in the early 60's I believe.