The ZX Printer was made to work with both the ZX81 and the Spectrum (or rather, the Spectrum was made to work with the ZX81's printer!), but these computers have different expansion ports.
What I find puzzling is that although the lines have been moved around, the printer works on both machines.
For example, the notch that makes sure you don't insert something upside down is #5 on the Spectrum, and #3 on the ZX81. So the offset is 2. That makes sense, as
A3 and the data lines are all shifted along by two. Same deal with
/M1. But then, for example
A11 has been moved way over the other side. And
/BUSREQ is now
+12V, I can picture that being dangerous.
How could devices like the ZX Printer keep working on the later computer? Why did they "move stuff around"?