I Am Missing Something In .50cal


"To Make One .50 Cal Bullet With Armored Insert"
Bought this at a gun show in Columbia SC for $40. Its missing #13 out of 18. The first 6 are elongating the cup, 7 - 11 is making the point, 12 is a .355 cal lead ball, 13 missing, 14 steel core, 15 everything in the jacket, 16 cannalure, 17 and 18 boattail. All put together with brass screws.
So whats #13 then? (lead sleeve?) And where can I find an extra? Thanks for any help. wolf