Thanks, everyone, for looking at my photos and explaining about the
square vs. round regulator.

That information helped when I spoke to Van Cafe today and made an
appointment to have the regulator replaced.

Rick at Van Cafe explained that, yes, if my set-up was unusual, they
may have retained my square regulator rather than make the changes
necessary to adapt to the round regulator they generally
install.  (That said, the square regulator looks newer than I
remember from before they replaced the tank, so maybe they found one
in stock and decided to use it.)

We did not discuss what they'll charge me, but I'm not too worried
about that.  For one thing, Van Cafe has always been very good about
supporting their work.  And anyway, I wouldn't expect them to cover a
defective regulator after a year and a half.

I'll report back after the repair next week.