I'm studying the design and construction of viewports for vacuum systems. Usually, the window is bonded to a welding ring, which is then welded to a stainless flange. (See the cross section below for an example.) I have encountered several synonyms that seem to be used interchangeably: adapter, sleeve, and stub. I'm looking for the proper name for this component so that I know what to ask for when contacting potential vendors, because I'm having a hard time finding these items in catalogs.
The red boxes indicate the welding ring; (source)