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mattrom, I don't believe U as abbrev for "up" is common in American crosswords (though I hardly ever try them any more), or in my cryptic experience. I agree those clues are a little strange, though fortunately not too hard to figure out.

I’d say it’s uncommon, although I see that it’s used for up in physics. Hm.
I’ve sometimes used E and F for empty and full because I see them on fuel gauges. Do you think that’s acceptable?

Thanks, Geeker and Aristo, for your replies. Aristo, I guess I would accept anything I can find in a dictionary.
Oddly, Collins has E for empty under American English, but not F for full.
I've drawn a blank for the "common element", but perhaps the penny will drop later.