You're right, it must be solid silver. I polished part of it, and underneath there is no sign of plate being worn off, even on the back of the bowl:

I'm not used to finding solid silver in a mixed lot of silver plate, so I wasn't thinking along those lines. But it also makes sense that it would be German.
I bought an old chest with four large drawers outfitted with "English racks." Three drawers were filled with older German WMF silver plate, but the fourth drawer was a jumbled mix of patterns and manufacturers. This is the first piece out of the mixed drawer that I've tried to identify. And there is a second, very similar soup ladle, also marked with "[13]"; however, it's very banged up:




Good grief, each one of these weighs almost half a pound!
Thank you for your help. I've gone from being bored with these to thinking they're pretty cool. And I wonder what else is in this drawer...