Thank you.
The pictures on the instructions to this gizmo do show a spring-like rubber thing as the ‘regulator’.
We’ve seen a restrictor before, so I guess we’ll pull that out first - if there is one - and see what happens.
Plumbing has changed in the last 20(?) years of living with restrictors.
First designed only to lower water flow, without regard to temperature or pressure, modern plumbing can regulate excessive changes in temperature and still provide enough pressure for a strong shower.
If you are just replacing the shower head, then look for a plate with holes in it that limit the amount of water that can pass.
If all you see is a spring loaded diaphragm, try to install it and see how you like the pressure.
If too low then start pulling pieces out slowly so you don’t break the shower head itself. There can be springs used to keep the head in place verses springs used to move the regulator.
Look up the brand and model on YT and see if someone has already made a video on how to remove it.