Looks like the basic twist-on tube shield.
You can get them in [raw aluminum, chrome plate, colored, etc.](https://www.tubesandmore.com/search/node/Tube%20shield) I can’t guarantee they will all swap in and fit the same, but most do.
It’s not required. The shield can held reduce some interference and it works as a tube retainer, but it can also be a source of mechanical noise.
You should be able to get them on ebay. Found some replacement tube covers for my Hiwatt
Looks like the basic twist-on tube shield. You can get them in [raw aluminum, chrome plate, colored, etc.](https://www.tubesandmore.com/search/node/Tube%20shield) I can’t guarantee they will all swap in and fit the same, but most do. It’s not required. The shield can held reduce some interference and it works as a tube retainer, but it can also be a source of mechanical noise.