These are pretty simple, a tube end cap. An item that everyone needs all the time. They are 1/8" thick plate and cut about 3/16" narrower than the tube they are intended for to help create a flush finish once welded. Now you can stop saving the drop every time you use a hole saw!
WARNING: This product can expose you to chemicals including lead, which is known to the State of California to cause cancer. For more information, go to www.P65Warnings.ca.gov.