Final note: The vacuum check system may extend all the way back to the 70's. I'm asking around to find out more....
I think you will find that doing a vacuum check to ensure that the seals on submersible enclosures are functioning correctly goes back a very long way. I spent a large chunk of my life as a Submarine Engineer Officer in the Australian Navy. The last thing we did before going to sea and diving after an extended period in harbour for maintenance was to close all the hatches, pull a vacuum inside the submarine and monitor over a period of time to ensure that none of the seals on hatches, periscopes etc were leaking air into the boat. This has been a standard maintenance procedure in submarines for a long time. If a vacuum check is good to keep me dry when the submarine dives (water in the people tank is not a good thing ) it's good enough to ensure my camera stays dry when I take it diving.
I think Retra's web site was a little out of date when I ordered my Retra Pro. At that time the angled adapter was offered as an optional extra and I ordered mine without it. When I received it, it was fitted with the angled adapter.
The angled adapter is good in that it allows you to align the LSD so that the aiming light in the strobe is shining on the centre of the light collector in the LSD. This ensures that the beam from the aiming light is a bright as possible and accurately shows where the beam from the flash will fall. The only downside I found was that because the angled adapter is clear to allow you to align the strobe's aiming light on the light collector, it does allow some light from the strobe to escape. Depending on strobe position this can allow escaping strobe light to light the background of your shot. For example:
Although it can work to your advantage as shown in these two shots, in general it doesn't.
However it is easy to fix the strobe side leakage problem. Once I had determined the optimum position for the LSD on the strobe I put an alignment mark on the LSD and the strobe and covered the clear adapter with duct tape.