I actually saw this bottle in the museum. Beautiful place, Speyer!
Just a few kilometers up the Rhine River, on the western side, are a series of oxbow lakes and sloughs, swamps and marshes that make up a huge nature reserve. My German friend had a canoe and we went out there many times and explored and discovered quite a bit things, both manmade, ancient and new, and natural.
We went out on the Rhine once, and canoe bobbed and swayed by the many ships and big barges going up and down. Too much and we got into another lake!
Sounds like a fun time!