Swapped CRT and a ?

[Dan Starnes]

About 18 months ago I located a NOS 21XP4 Crt. Figured I would used a snowy cold saturday to swap it out from the old Crt that had grown tired. This is on my Admiral that I have had for several years. Took about an hour to get it all swapped out and when I went to test I was amazed at how well it was set. So I worked on centering and the like when I noticed I had forgotten to place a spring tensioned metal band on the neck of the Crt (pictured, and sorry cell phone pics) When I placed it on the new Crt the images just went to white. I had raster, took it back off and it worked again. The crazy thing is, if I had put it on before I tested the set I would have never known what the problem was. Question is, what is the band and what does it do? This set used to be my daily viewer and since it works so well again, just might be watching it a whole lot more.