The rubber L shaped piece likely came with the barracuda - IMO - garbage.
The "weird appliance" from the barracuda - IMO also garbage, it's an automatic safety bypass gadget that triggers when the suction increases, in 3 years of running my barracuda, it has never been used and I have never had issues.
I think it's supposed to go to bypass when something blocks the diaphram - like a washcloth. Since my kid bathes in the bathtub, we don't keep excess washcloths inside our pool.
I did try to use it, however, it kept triggering at all 3 settings when the cuda was working fine, so I filed it under "extra pool crap".
I also can't see my cuda sucking itself to the wall unless it miraculously wears off all the bumps on the bottom in 1 or 2 days.

My skimmer plate adapter has the L built right in so my vac hose comes out through the skimmer opening, however, you can connect your hose straight down onto a non L unit and have the hose come out of the top of the skimmer, over the wall and into the pool that way - works just as well.

Your cuda also came with a short 4" piece of female to female hose adapter - may be right on the cuda itself at the neck, if it's there, remove it, start with your hose sliding the female end over the cuda neck, keep the 4" stub to adapt to the different things at the skimmer end, for instance, you will need it if using the skimmer plate to connect the hose, however, if you plug the hose directly down into the hole in the bottom of the skimmer, you will want to use the male hose end and set the 4" stub aside.