what's this: a square 'U'

In the Symbols Palette (shortcut "Z"), yes.
But also in the Special Characters (Ctrl + T + F2) -> Musical Symbols -> String Techniques, as in my attached picture above.
The big advantage of Special Characters is that you can edit the size of the symbols. But in this case, it is true that it does not seem necessary for this symbol "Turned down bow".

If one wishes to use this specific rare symbol, indeed, that's where you'd find it. If you have a score where a composer or editor used it a lot, you can then copy it to your own palette for easy reuse. Also, note that to open the Symbols portion of the master palette, just press "Z", and you can also find the symbol by simply typing "bow" into the search box.

Without seeing the full score itself, though, I suspect it's really meant to be a standard downbow symbol and the engraver just placed it upside down for some reason. Apparently a number of French publishers back in the 1800's did this. In which case, much better to simply replace it with the standard downbow symbol to avoid confusing your readers the way it's confusing us here too!