On a Williamson UL PP amp, the screen is at about the same DC potential as
the anode (as measured with a DVM). I thought that the screen had to be at
a lower potential for normal PP operation.

What's up with that?

A reasonable question. No doubt you'll get a multitude of answers from the
folks here, but here's a simple one:

If you tied the screen directly to the plate, you'd have - a triode. What the
UL tap on the transformer does is put some signal on the screen, but not the
full signal that appears on the plate, so the tube runs somewhere between a
triode and a pentode. It's actually a lot more complex than that, but that's
the basic idea.

Note that some tubes cannot be operated in this mode due to screen voltage
limitations. The 6146 is an example. If you wanted to run these in UL mode,
you'd need a separate winding on the OP trans.