I'm sure your first question was the same as mine -- why would a WSL cap be made in Germany? I don't think a British merchant mariner would crown his head with a German-made cap even in 1912, and certainly not after 1914. Even if the cap were German, it would have had to bear the label "Made in Germany," which was required on all goods imported to the UK from Germany after 1887.
There is a page
here with a better image of the Holter-Uniformen mark. (WARNING: The link is to a site that sells reproduction Nazi equipment, as used by reenactors. It is not a neo-Nazi site but it contains Nazi symbols and may therefore be illegal in Germany.) The mark shown here seems to be printed with a modern typeface, whereas the original is hand-drawn.
The clear material would have to be celluloid if this is pre-1945 -- there's no easy way to tell if it's vinyl or polyethylene or HDPE unless you know those materials by touch.
Now, we are left with one intriguing possibility, but unfortunately it also doesn't pan out. Since this was made in Germany I thought perhaps it was a prop for the 1943 film. However, stills from that film show that the oak leaves on the cap extend just a little further, almost touching, and there is no crown.
I can't give a 'verdict' but a guess is that it was made by a company that makes reproductions of German WW2 equipment, and meant for sale to people who know the Holter firm (which really did make a lot of Nazi-era uniforms).