Richard Clark wrote:
"This conforms to my experience with many plumbing designs on the
microwave bench."

Good. Then, Richard Clark must also be familiar with the grooved
circular flange used in conjunction with a smooth flange to join
waveguide segments. This groove isn`t just used to hold a neoprene
gasket. It is also used as an electrical choke to keep the microwaves
within the pipe. It is approximately a 1/4-wave choke and its high
impedance across its open-circuit helps foil the wave escape. If virtual
open-circuits didn`t work, the "choke-flange wouldn`t work either.

Best regards, Richard Harrison, KB5WZI