The docs of the pcf8574 and pcf8574a datasheets show the 7 bits and R/!W. It made enough sense I could figure out how to get the test circuit working okay. I'd seen the Wire docs previously so maybe it didn't confuse me as much as I thought it would.
These devices have four address bits that are hardwired and unchangeable, three address bits that you jumper to three pins, and then the R/!W bit.
What did throw me though, I thought I ordered pcf8574 and got pcf8574a. The only difference between these are the constant value of the four hardwired address bits. And the silkscreening on some packages is very very dim, so I didn't see the problem when I made a test circuit. I had to write a loop to try all addresses...